A CMOS Instrumentation Amplifier With 90-dB CMRR at 2-MHz Using Capacitive Neutralization: Analysis, Design Considerations, and Implementation

Transcription

1 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I: REGULAR PAPERS, VOL. 58, NO. 4, APRIL A CMOS Instrumentation Amplifier With 90-dB CMRR at 2-MHz Using Capacitive Neutralization: Analysis, Design Considerations, and Implementation Apisak Worapishet, Senior Member, IEEE, Andreas Demosthenous, Senior Member, IEEE, and Xiao Liu, Member, IEEE Abstract The benefits of using current feedback in instrumentation amplifier (IA) design are well known. In this paper, we analyze the mismatch mechanisms, both random and systematic types, which influence the common-mode rejection ratio (CMRR) performance of the local current feedback IA topology. We derive analytical expressions for the common-mode gain frequency response due to random mismatches (transconductance, drain-source conductance and parasitic capacitance) and verify the integrity of the analysis through simulation. To address the systematic mismatch in the drain capacitance of the input pair transistors, we employ capacitive neutralization and verify its effectiveness in practice from the fabricated IA chip samples in a m CMOS process technology. The measured average common-mode gain improvement for the 20 fabricated samples employing our neutralization technique is about 20 db at 2 MHz ( 3 db bandwidth). When taking into account the differential gain response (33.7 db), the average CMRR of the neutralized IA at2mhzexceeds90db.theiaoccupiesanareaof0.068mm and dissipates 0.85 mw from a 3-V power supply. The circuit is intended for a wideband bioimpedance spectroscopy application. Index Terms Capacitive neutralization, CMOS, common-mode gain, component mismatches, high-frequency, high CMRR, instrumentation amplifier (IA), local current feedback, medical applications, wide bandwidth. I. INTRODUCTION I NSTRUMENTATION amplifiers (IAs) are very important circuits in many sensor readout systems where there is a need to amplify small differential signals in the presence of large common-mode interference. Application examples include automotive transducers [1], industrial process control [2] [4], linear position sensing [5], and biopotential acquisition systems [6] [11]. We have a particular interest in the design of integrated instrumentation for medical impedance imaging Manuscript received February 20, 2010; revised June 28, 2010; accepted August 17, Date of publication November 15, 2010; date of current version March 30, This work was supported in part by the UK Engineering and Physical Research Council (EPSRC) under Grant EP/E029426/1 and Grant EP/G061629/1, and in part by the British Council Researcher Exchange Programme. This paper was recommended by Associate Editor J. S. Chang. A. Worapishet is with the Mahanakorn Microelectronics Research Centre and Department of Telecommunication, Mahanakorn University of Technology, Bangkok 10530, Thailand ( apisak@mut.ac.th). A. Demosthenous and X. Liu are with the Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K. ( a.demosthenous@ee.ucl.ac.uk; x.liu@ee.ucl.ac.uk). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TCSI using bioimpedance measurements [12], [13]. In clinical applications the use of bioimpedance imaging, also known as electrical impedance tomography (EIT) [14], offers advantages over other medical imaging techniques. Unlike computerized tomography (CT) and X-rays, EIT does not emit ionizing radiation, and unlike magnetic resonance imagining (MRI), EIT is silent, highly portable, and inexpensive. EIT works by reconstructing the differences in electrical conductivity inside a body. In a typical bioimpedance measurement system, a differential alternating current is applied through a pair of surface electrodes to the body tissue and the resulting voltages are picked up by another electrode pair and amplified for further processing [15]. The front-end amplifier is required to have high input impedance to avoid part of the injected current shunted into the recording electrodes which would cause errors in the measurement; hence the requirement for an IA. The main common-mode interference in bioimpedance measurements occurs at the working frequency and is produced by the current injected into the body to make the measurements [16]. In the case of EIT, the differential signal measured between adjustment pair of electrodes can be as small as a few tenths of a microvolt V whereas the common-mode interference can be in the hundreds of millivolt (mv) range. For imaging of cancer biomarkers which is our target application, it is necessary to measure bioimpedance over a wide frequency range (10 khz to 1 MHz) and in multifrequency mode (bioimpedance spectroscopy). Furthermore, it is required that the minimum detectable input signal be as low as 20 V. The need for wide bandwidth (BW) operation dictates that the IA should have high common-mode rejection ratio (CMRR) at high frequencies. CMRR is defined as the ratio of the differential gain over the common-mode gain [17]. To obtain good accuracy in the measurement, our specification for the IA requires a minimum CMRR of 80 db up to 2 MHz ( 3 db BW). However, to the best of our knowledge, neither off-the-shelf monolithic IAs nor those reported in the literature meet this specification. The CMRR of high-performance IAs such as the AD8221 [18] is 80 db only up to about 100 khz. Although commercial high-speed differential receiver amplifiers such as the AD8129/AD8130 [19] feature high CMRR at high frequencies, their current consumption is very high ( 10 ma). In addition, we are aiming for a low-power, fully-integrated, system-on-chip solution for the targeted bioimpedance spectroscopy system. The design of alow-power IA with high CMRR at high frequencies is a very challenging task /$ IEEE

2 700 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I: REGULAR PAPERS, VOL. 58, NO. 4, APRIL 2011 There are two basic approaches to the design of an IA: resistive feedback (e.g., 3-opamp topology [11]) and current feedback [20]. In the case of resistive feedback, the CMRR is limited by the degree of matching of the resistors in the feedback network; only balancing technique is employed. In the case of current feedback, higher CMRR performance is achieved because both isolation and balancing techniques are employed. In addition, the current feedback approach offers a higher operating BW than resistive feedback [20]. A typical current feedback IA consists of a resistive-degenerated input transconductor (i.e., converter), a resistive-degenerated output transconductor, and one or more high gain feedback loops. If a single feedback loop is applied around both transconductors, the IA is classified as direct [6] or indirect [21] current feedback. In the direct current feedback IA the two transconductors are stacked and this limits the input common-mode voltage range and the minimum supply voltage. If two isolated local feedback loops are used, one around the input transconductor and one around the output transconductor, the IA is classified as local current feedback [22]. Both the direct and indirect current feedback IA topologies are subjected to a number of parasitic poles associated with each of the stages around the loop. As a result, this complicates the frequency compensation and poses a limitation on high-frequency operation. On the contrary, in the local current feedback IA topology, each local loop contains a smaller number of internal parasitic poles and thus, this topology potentially offers a higher operating BW for a given current consumption. For these reasons we have chosen the local current feedback IA topology implemented with a current mirror load (drain load) in the input transconductor [7]. One advantage of the current mirror load over the resistor load implementation [22] is insensitivity to the input offset voltage of the sensing (or loop) amplifier connected across the input and output nodes of the current mirror load. In addition, the current mirror load provides a large local loop gain due to its high-impedance output node. As a result, the sensing amplifier can be single stage with relatively low gain. This yields a consequent benefit to stability and high BW operation due to reduction of the parasitic poles around the loop, hence a simple and power-area efficient implementation. Analysis of the CMRR performance of the local current feedback IA has been limited to low-frequencies [23]. In this paper, we analyze the mismatch mechanisms (both random and systematic types) that influence the CMRR performance of the local current feedback IA from low-to-high frequencies and design the circuit to meet the CMRR specificationofminimum80dbupto2mhz.toaddressthesystematic mismatch in the drain capacitances of the input pair transistors, we use capacitive neutralization and verify its effectiveness through simulation and measurements from the fabricated IA chip samples in a m CMOS process technology. The remaining sections of the paper are organized as follows. Section II presents the common-mode gain frequency response analysis of the IA s input stage for random mismatches (transconductance, drain-source conductance and parasitic capacitance) taking into account the dominant poles and zeros of the common-mode voltage transfers. Analytical expressions for the feedback current for each mismatch parameter are derived, Fig. 1. Simplified schematic of the local current feedback IA with current mirror load in the input transconductor. yielding the overall common-mode gain response of the IA. Section III discusses the expressions in terms of their frequency characteristics and their relative contribution to the overall mismatch feedback current. Section IV examines the systematic mismatch in the drain capacitances of the input pair transistors and proposes the use of capacitive neutralization to mitigate this imbalance. The IA circuit implementation and design considerations are detailed in Section V. Simulated and measured resultsarepresentedinsectionvi,followedbyconclusionsin Section VII. II. IA COMMON-MODE GAIN RESPONSE ANALYSIS Fig. 1 shows the simplified circuit schematic of the local current feedback IA [7], [22]. The input transconductor stage uses a simple current mirror load (drain network) and current source biasing (source network). The sensing amplifier serves to exactly balance the drain currents of transistors and by adjusting the complementary currents and. A direct result of this is that the input differential voltage is forced across resistor and hence and of the input stage essentially acts as a unity-gain buffer. Similarly, the high gain amplifier balances the drain currents of transistors and in the output transconductor stage. Since currents and are exact copies of and, respectively, the output voltage appears across resistor. Hence, the dc gain of the IA is given by the ratio. Placing a capacitor in parallel with resistor creates a dominant pole, which sets the 3dBBWoftheIA. A. Analysis Formulation The common-mode gain characteristics of the IA due to random mismatches can be analyzed by focusing only on the input stage. This is because the mismatch effects of the sensing amplifier and the output transcondutance stage are greatly suppressed by the high gain of the local feedback loops. Fig. 2 shows the small-signal model of the IA s input stage where it is assumed that the output conductances of are much less than their corresponding transconductances. The voltages at the drain terminals, and, are sensed by the amplifier which drives the differential feedback current. The feedback path is via the source terminals and.inthefigure, and

3 WORAPISHET et al.: A CMOS INSTRUMENTATION AMPLIFIER WITH 90-dB CMRR AT 2-MHz 701 Fig. 2. IA input stage model for common-mode gain analysis. are respectively the transconductances and drain-source conductances of the input transistors and. are the transconductances of the drain transistors and. are the output conductances of the current sources and. All parasitic capacitances are included to allow a study of the high-frequency mismatch characteristics. and are respectively the gate-source and gate-drain capacitances of the input transistors and,and and are respectively the total capacitances of the source and drain terminals, including those from the input and load/source transistors as well as the amplifier stages that are connected to the terminals. To facilitate the analysis and description, the resistors in the model, except, are expressed by the conductance parameters. In order to systematize the common-mode analysis, we have chosen to introduce mismatches via the following definitions: the relative mismatch between the drain capacitance of the drain network. The signs for each of the mismatch parameters defined above result in cumulative contributions. A set of equations governing the common-mode gain characteristics of the IA can be formulated by first applying KCL at the drain and source terminals of the input stage. Next, to obtain the equations for the common-mode signal responses, we take the sum between the drain equations at and, and between the source equations at and. Similarly, to obtain the equation for the differential-mode signal response, we take the difference between the drain equations at and, and between the source equations at and. These sum and difference equations of the KCL node equations enable us to understand the underlying mechanism that leads to a finite common-mode gain due to component mismatches. In particular, the sum equations will be employed to determine the common-mode voltages of the IA. If there are mismatches, the common-mode voltages will give rise to differential current injections into the circuit. Subsequently, the difference equations will be employed to determine the circuit response, and hence the finite common-mode gain of the IA can be computed. In order to simplify the analysis, the following approximations are applied to those sum and difference equations: a) b) c) d) e) f). With reference to Fig. 2, are the transconductances of transistors. and are respectively the conductances and the capacitances associated with terminals. Note that the symbols,,and represent the nominal value of their corresponding parameters. Also, and are respectively the common-mode voltages and their nominal voltage. is the relative mismatch of the parameter under consideration. Consequently, the equations governing the common-mode signals, and,ofthe input stage are given by (1) where the symbols,,,,,, and represent the nominal value of their corresponding parameters. With reference to Fig. 2,,,,and respectively represent the relative mismatches between the transconductance, drain-source conductance, gate-source capacitance, and gate-drain capacitance of the input transistors and.also, and are the relative mismatches between the output conductance and capacitance of the source network, represents the relative mismatch between the transconductance of the current mirror transistors, and is (2) and those governing the differential-mode signals,, and the feedback current,aregivenby (3) and (4) (3)

4 702 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I: REGULAR PAPERS, VOL. 58, NO. 4, APRIL 2011 (4) The common-mode gain characteristics of the IA are also dependent upon the sensing condition of amplifier in Fig. 2. For a balanced sensing scheme, the amplifier is of a differential type where its input terminals sense the voltage difference between the drain terminals of the input transistors. Typically, the sensing amplifier together with the high impedance at the output of the drain active load provide a high differential loop gain within the operating BW, whereas the common-mode loop gain is small ( 1). Following this, we have,i.e., a differential-mode virtual ground condition. Note that to include the offset voltage of the sensing amplifier, the relation can be modified to where represents a constant offset voltage. It is also possible for an unbalanced or single-ended sensing at the drain terminals of the input transistors. However, this scheme leads to systematic common-mode gain responses hence it is not of practical and analysis interest. B. Qualitative Description of the Common-Mode Responses To seek a qualitative insight from the set of equations in (1) to (4), we start by showing the common-mode and differentialmode equivalent half circuits of the IA s input stage. These are described by the common-mode equations in (1) and (2), and the differential-mode equations in (3) and (4), and are shown in Fig. 3(a) and 3(b), respectively. By virtue of superposition between the common-mode and differential signal responses, we can deduce the mechanism that gives rise to finite commonmode gain responses due to component mismatches in the IA. Consider the common-mode half circuit of Fig. 3(a). Upon the presence of the common-mode input, the common-mode voltages and are deviated from the quiescent operating points, and these can be determined from (1) and (2). As a consequence, the incremental common-mode voltages produce incremental common-mode currents through the admittances of the resistive and capacitive circuit components. If mismatches exist, the incremental common-mode currents between each pair of components will be slightly different, yielding nonzero differential current sources,,,,,,,and in the differential-mode half circuit of Fig. 3(b), as defined in (3) and (4). This implies that the differential mismatch currents will follow the frequency characteristics of the common-mode voltages and their developed or controlled admittances. These mismatch current sources in the differential circuit of Fig. 3(b) produce the differential voltages and, as well as the differential currents, particularly the feedback current as a result of the high gain negative feedback mechanism for differential signals at the input stage. By using (3) and (4),, and can be determined. The mismatch current isalsofedtotheoutput Fig. 3. (a) Common-mode equivalent half circuit. (b) Differential-mode equivalent half circuit (capacitance omitted). transconductor stage, yielding a finite output voltage hence a finite common-mode gain in the IA. C. Derivation of Analytical Common-Mode Responses and Upon solving (1) and (2), it can be shown that with the dominant pole/zero approximation [24], the common-mode drain and source voltages, and, as well as the common-mode gate-source and gate-drain voltages, and, all exhibit an -domain transfer characteristic of the form where and respectively represents the dominant zero and pole, and is the dc gain. Since the common-mode voltages share identical pole locations, by solving (1) and (2) and applying (5), the dominant pole magnitude is derived as given in Table I. By following the same procedure, the dominant zero magnitude for each common-mode voltage is derived and these are also given in Table I. Further approximation of the voltage characteristics can be obtained by recognizing that it is typical for an IA to have the overall BW set by one single dominant pole. For the local current feedback IA of Fig. 1, this is set by the time constant at the output transconductor stage, where (5)

5 WORAPISHET et al.: A CMOS INSTRUMENTATION AMPLIFIER WITH 90-dB CMRR AT 2-MHz 703 TABLE I DC GAIN AND DOMINANT ZERO AND POLE MAGNITUDES OF COMMON-MODE VOLTAGE TRANSFERS TABLE II EXTRACTED SMALL-SIGNAL CIRCUIT PARAMETERS OF THE DESIGNED IA TABLE III CALCULATED DC GAIN AND POLE/ZERO MAGNITUDES OF COMMON-MODE VOLTAGES the should be somewhat less than the pole frequency (in Hz) in Table I. Thus, for the analysis within the BW of the IA, the pole associated with the common-mode voltages can be omitted. From Table I we observe that the transconductances and are associated with the zero magnitude of the source voltage, gate-drain voltage, and drain-source voltage, similar to the expression. As a result, these zeros should be located beyond the BW of the IA and can also be omitted. On the other hand, the much smaller conductances and are associated with the zeros of the drain voltage and gate-source voltage. Consequently, the dominant zeros of and tend to be located at frequencies below the BW of the IA and hence, must be included in the analysis. Using the extracted small-signal circuit parameters of the designed IA (see Section V) in Table II and the analytical expressions in Table I, the calculated magnitudes (in Hz) of and are given in Table III. When compared with the 3 db corner frequency (2 MHz) of the IA, the locations of the zeros follow the discussion above, thus validating the approximation. Based upon the above pole/zero approximations of the common-mode voltages, the mismatch current sources in the differential-mode half circuit of Fig. 3(b) can be determined. Then, the voltages, and the feedback current can be derived. In this case, we can further assume that the poles/zeros associated with the differential-mode circuit are at frequencies beyond the BW of the IA and can be neglected. This is justified by the fact that the degeneration resistance in the differential-mode circuit is much smaller than the inversed conductances and, which determine the dominant pole of the common-mode voltages. Solving (3) and (4) and applying the condition,,the approximated closed-form expressions of the current transfer with, are summarized in Table IV for each mismatch parameter. Note from Table IV that the effect associated with the offset voltage of the sensing amplifier is suppressed by the factor due to the use of a current mirror as the drain load. The common-mode gain frequency response of the IA for each mismatch parameter in Table IV is obtained by The sum of all these common-mode gain frequency responses yields the overall common-mode gain frequency response of the IA. III. DISCUSSION OF THE ANALYTICAL EXPRESSIONS A. Frequency Characteristics For frequencies well below the zero frequencies of the common-mode voltages and, the current transfers in Table IV due to the (trans)conductance mismatches, (6)

6 704 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I: REGULAR PAPERS, VOL. 58, NO. 4, APRIL 2011 TABLE IV APPROXIMATED FEEDBACK CURRENT TRANSFERS DUE TO MISMATCHES AND OFFSET VOLTAGE,,and remain constant, following the characteristics of the common-mode voltages and the (trans)conductances that are practically constant over the range. Over the same frequency range, the current transfers due to the capacitance mismatches,,,and, exhibit linear frequency dependency, following the characteristics of capacitive admittances which increase linearly with frequency. As we approach the zero frequencies of the commonmode voltages and, which are located below the BW of the IA (see Table III), their voltage magnitudes start to rise linearly with frequency. Because and control the transconductances and, respectively, the current transfers due to and start to exhibit linear frequency dependency. Similarly, because and develop across and, the current transfers due to and start to exhibit quadratic frequency dependency. However, the current transfers due to the conductance mismatches,,and the capacitance mismatches,, remain similar to their corresponding low-frequency characteristics. This is because the characteristic zeros of their developing voltages (,,or ) are located well beyond the BW of the IA (see Table III) and thus, the voltage magnitudes remain practically constant. Another important indication from the analytical expressions in Table IV is the fact that, at high frequencies approaching the BW of the IA, all the capacitive mismatch currents are independent of the output conductance of the current source. This is remarkably opposite to the low-frequency common-mode responses where should be low to enable large suppression of the mismatch errors. This implies a possible omission of the cascode transistor arrangement in the feedback current source. As a result, potential benefits in terms of BW and stability due to fewer nondominant poles around the feedback loop can be gained. Other benefits include supply voltage reduction and larger output swing due to a smaller voltage headroom requirement. B. Relative Contribution of Mismatch Currents Let us now investigate the relative contribution to the overall mismatch current for each of the mismatch components. This can be of great importance when designing and optimizing the IA for a high CMRR at high frequencies. Only the effect of random mismatches is considered in this section. The systematic mismatch will be discussed in Section IV. We base our observations again on the expressions in Table IV, noting the underlying mechanism that the mismatch currents are dependent upon the admittances and their developing or controlling common-mode voltages. This implies that the mismatch current will be larger if the admittance and/or the common-mode voltage are higher, and vice versa. Another parameter that must be considered is the multiplying factor associated with the mismatch currents that produce nonzero differential voltages in the differential-mode half circuit of Fig. 3(b), particularly the source voltage,whenflowing to the source feedback terminals. This will produce additional current flowing through the degeneration resistor which further increases the mismatch current, yielding the factor.since is typically small to allow a large differential gain and low noise in the IA, the factor can be somewhat larger than unity. This can greatly enlarge the overall random common-mode gain of the IA, and must be taken into consideration in the design. At low frequencies, the mismatch currents due to the (trans)conductancesdominatesincetheadmittancesofthecapacitance mismatches are comparatively negligible. By considering the relative values between the (trans)conductances,, and, and their developing or controlling voltages from the transfer responses in Table I, as well as the factor associated with their mismatch expressions in Table IV, we may deduce that the mismatch components, and dominate at low frequencies, where their relative contributions depend upon the actual values chosen for a target design. The above discussion and the expressions in Table IV also indicate suppression of the (trans)conductance mismatch currents by the drain-source conductance of the input transistors, and/or the output conductance of the current source. This typifies the isolation characteristic of the current feedback IA, where small and are generally recognized to provide improvement on the low-frequency CMRR performance. As frequency increases, the capacitance mismatch currents increase and their effect are no longer negligible. Again, by considering the relative values between the capacitances,, and, and their developing or controlling voltages from the transfer responses in Table I, as well as the factor associated with their mismatch expressions in Table IV, it follows that, given the same mismatch error, the mismatch is dominant. It should be noted that even though the high-frequency characteristics of the mismatch currents due to and become quadratically-dependent as describedinsectioniii-aand Table VI, this appears quite close to the BW of the IA. As a result, the effect is quickly attenuated and the contributions from and to the overall mismatch current are still relatively small. Another important effect that requires attention at high frequencies is the characteristic zeros in the voltage transfers and. As a consequence, the mismatch currents

7 WORAPISHET et al.: A CMOS INSTRUMENTATION AMPLIFIER WITH 90-dB CMRR AT 2-MHz 705 Fig. 4. Schematic diagram of the IA using neutralization capacitor to balance the drain capacitances of the input pair transistors. due to and become linearly dependent against frequency. Similar to what was described before, however, the effect is quickly suppressed since the zeros are located near the BW of the IA. IV. SYSTEMATIC MISMATCH AND CAPACITIVE NEUTRALIZATION Upon examining the IA schematic of Fig. 1, we note that there exists a systematic mismatch in the drain capacitances and (see small-signal circuit in Fig. 2) due to the inherent asymmetrical topology of the current mirror load ( and ). With reference to the circuit in Fig. 1, the expressions for the total drain capacitance are given by (7) (8) where is the total common-mode capacitance at the input of the differential sensing amplifier,and are the gate-source capacitances of the drain transistors. Also, and are respectively the drain-bulk capacitances of the input transistors and the drain transistors. By assuming perfect matching of components, i.e., and, the mismatch parameter is given by Since the gate-source capacitance at the drain terminals typically constitutes a large proportion of the effective drain capacitance, the mismatch parameter can be considerably larger than the case with only the random mismatches. Hence, the effect of the systematic drain capacitance mismatch can dominate the common-mode gain response of the IA at high frequencies. (9) A simple means to suppress this adverse effect is to insert a neutralization capacitor at the terminal of the current mirror load, to reduce the parameter in (9) by balancing the capacitances and. To allow good tracking against process and temperature variations, the neutralization capacitor is implemented by the gate capacitance of a MOS transistor in nonsaturation operation as shown in Fig. 4, since this is of a similar type as at the terminal. Assuming the MOS total gate capacitance under nonsaturation operation is, and the MOS gate-source capacitance under saturation operation is, it can be shown that, for the same length as the current mirror transistors, the width of the neutralization MOS capacitor is given by (10) where is the width of the current mirror transistors. With the use of the capacitive neutralization to suppress the systematic mismatch, the high-frequency CMRR can be dramatically improved, practically at no cost to other performance. However, this improvement is limited by both the random mismatch and the inherent asymmetrical circuit configuration of the drain active load in the IA. As indicated by the developed expression in Table IV for, one may increase the product and/or the transconductance in order to further suppress the parameter. This can be obtained by trading off power consumption for a larger or, and/or trading off noise for a larger, etc. Note however that, with reference to Fig. 9 (see Section VI-A) the effect of even with 5% residual mismatch gives a lower common-mode gain compared to the sum of the other capacitance mismatches, each with 1% error. This relative contribution thus should be carefully taken into account in the tradeoff. It should be noted that the capacitive neutralization in this work is entirely different from the neutralizing capacitor technique for wideband amplifiers under a differential signal excitation in [25] and [26], which makes use of the bridge capacitors

8 706 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I: REGULAR PAPERS, VOL. 58, NO. 4, APRIL 2011 TABLE V IA TRANSISTOR DIMENSIONS scheme to compensate for the input/output parasitic coupling capacitance in the transistors. Although one may employ the dynamic element matching (DEM) to suppress the mismatch [27], it typically requires a high clock frequency beyond the BW of the IA. Another point worth discussing is the fact that the systematic mismatch arises from the use of a current mirror as the drain load. An obvious means to avoid such mismatch is to change the load to a current source type. However, this would require additional circuit complexity and power consumption, because a common-mode feedback circuitry would be necessary for setting up the dc drain voltages. Hence, the current mirror load equipped with the neutralization capacitance isamoreefficient solution in terms of simplicity, compactness and low power requirement. The effectiveness of the neutralization technique will be demonstrated in Section VI. V. CIRCUIT IMPLEMENTATION An IA using the local current feedback topology in Fig. 1 but with pmos input transistors (to eliminate the body effect in the input pair transistors) was designed, simulated and fabricated using the m austriamicrosystems CMOS process technology [28] for a differential gain of 50 V/V (set by ), 3 db BW of 2 MHz, and 3 V supply voltage operation. The bulk terminal of each nmos transistor was connected to the negative supply (0 V) while that of each pmos transistor was connected to its source terminal (n-well technology). The simulations and layout were carried out in Cadence Analog Environment using the design kit provided by the foundry. The full schematic of the designed IA is shown in Fig. 4 and its transistor dimensions are listed in Table V. The sensing amplifier in the first stage was implemented by a simple transconductor ( A, ) whose output currents are copied to the input and output transconductor stages. Although the use of cascoding can reduce the capacitances and the conductances at the source terminals, for our specific IA design, their relative impact on the CMRR is negligible as suggestedlaterinfigs.6to8(seesectionvi-a).forthisreason together with the high BW requirement, no cascode transistors were used for the feedback current sources ( and ) to ease the design at high frequencies with no impact on the high-frequency CMRR performance as indicated by the analysis. The output stage merges the transconductor and sensing Fig. 5. A microphotograph of the fabricated chip with four versions of the IA. From left: using fixed nmos neutralization capacitor, using nmos variable neutralization capacitor, using pmos variable neutralization capacitor, and nonneutralized. The chip was designed and tested at University College London. amplifier into a single circuit, realized by a symmetrical CMOS transconductance operational amplifier with cascode transistors for high loop gain and high BW operation. The cascode transistor pairs, and, are biased by the external dc voltage sources (1.2 V) and (1.8 V), respectively. The dc level of the output voltage is set by the external voltage source (1 V). To allow testing at high frequencies, an on-chip 5 V pmos source follower (not shown) with an output impedance of about 50 succeeds the IA. The source follower does not affect the CMRR of the IA. The IA s input stage dictates noise performance. Since the BW of the IA extends to high frequencies (2 MHz), thermal noise dominates over flicker noise. Hence, using only the thermal noise contribution, the input-referred voltage noise of the IA can be calculated as (11) where is Boltzmann s constant, is the absolute temperature, and is the BW in Hz over which the noise is measured. To minimize the thermal noise contribution of resistor, its resistance must be set to a small value (here 400 ). Biasing the input stage transistors with a quiescent drain current of 37.5 A, to obtain a total input integrated noise in the referred BW (i.e., MHz) of less than 20 V (rms), we ended up with the transistor dimensions in Table V for and (the dimensions were fine tuned in Cadence) and with their extracted trans(conductance) and parasitic capacitance values in Table II. The sizing of the transistors in Table V was chosen to be relatively large for layout matching purposes (especially the nonminimum length). For good linearity, the maximum differential input signal range should be restricted to the value of the product (here 30 mv) where is the quiescent drain current of the input stage current source transistors and (see Fig. 4). Increasing the input signal much beyond this limit, would result in appreciable output harmonic distortion. The chip microphotograph is shown in Fig. 5. The layout employed common-centroid and interdigitation for matched devices. The layout area of the IA including routing and the output

9 WORAPISHET et al.: A CMOS INSTRUMENTATION AMPLIFIER WITH 90-dB CMRR AT 2-MHz 707 Fig. 6. Simulated (point lines) versus theoretical (solid lines) common-mode gain responses due to transconductance and conductance mismatches. The mismatch parameters were set to 1%. Fig. 7. Simulated (point lines) versus theoretical (solid lines) common-mode gain responses due to the capacitive mismatches and.themismatch parameters and were set to 1%. buffer is mm (the pmos follower occupies mm ). Four versions of the IA were implemented on chip (see Fig. 5) with identical component parameters and layouts. The only difference is the addition of the neutralization capacitor in three of the IAs for suppression of the systematic drain capacitance mismatch of the input pair transistors (see Fig. 4). One IA used a grounded fixed nmos capacitor, one used a variable nmos capacitor, and the other used a variable pmos capacitor, each with their drain and source terminals shorted. These MOS capacitors used the same length as the nmos transistors and in the current mirror load. The width of the fixed capacitor was initially selected in accordance with (10), and subsequently refined with the help of postlayout simulation. The variable MOS capacitors used a width of 30 m and a length of 5 m; the capacitance value could be altered by varying the voltage applied to the shorted drain-source terminal [29]. The purpose of the variable capacitors was to investigate the effect of the neutralization capacitance value on the overall common-mode gain of the IA. In total 20 chips were fabricated and tested with all samples working. VI. SIMULATED AND MEASURED VERIFICATIONS A. Simulated Versus Theoretical Characteristics We first consider the case when the IA is equipped with the neutralization technique. The small-signal circuit parameters of the neutralized IA s input stage were extracted as listed in Table II for verification of the theoretical derivations. To enable investigation of each of the mismatch effects via simulation, deterministic mismatches were included in the transistor-level schematic to set the trans(conductance) and capacitance mismatches (i.e., the factors ) in the circuit parameters in Table II. Moreover, to avoid interfering with the operating conditions of the IA, these were set up by adding resistors, capacitors, and independent current sources. Fig. 6 shows the simulated common-mode gain responses versus frequency for the (trans)conductance mismatches for 1% mismatch (i.e., ). Similarly, Figs. 7 and 8 show Fig. 8. Simulated (point lines) versus theoretical (solid lines) common-mode gain responses due to the capacitive mismatches and.themismatch parameters and were set to 1%. Both capacitive mismatches are plotted at two different values of the current source output conductance: and. the simulated responses for the capacitance mismatches for 1% mismatch. Also shown in the same plots are the theoretical common-mode gain responses based on the current expressions in Table IV and using (6). As seen from these plots, the simulated and theoretical common-mode gain characteristics are in good agreement up to frequencies beyond the BW (2 MHz) of the IA. This validates all the approximations taken to simplify the analysis in Section II and the accuracy of the analysis. As the frequency approaches the 10 MHz region, there are discrepancies. This is primarily due to the effects of the nondominant poles/zeros neglected in the analysis. Furthermore, in order to verify that at high frequencies the common-mode gain responses due to the capacitive mismatches and, are insensitive to the current source output conductance, the gain responses were also simulated with a 10 times smaller (modified with the use of ideal negative resistors). As seen in Fig. 8, at high frequencies, a smaller has negligible impact on the common-mode gain responses due to the and mismatches. However, at low-to-intermediate frequencies,

10 708 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I: REGULAR PAPERS, VOL. 58, NO. 4, APRIL 2011 Fig. 9. Simulated (point lines) versus theoretical (solid lines) common-mode gain responses due to capacitance mismatches. Shown is the response for the systematic drain capacitance mismatch, compared to the response of for1%mismatch.alsoshownis the response for a 5% deviation in the neutralization capacitance from its optimum value. asmaller helps to suppress the common-mode gains due to the and mismatches. At frequencies near the BW of the IA, the responses converge to the cases with larger. This conforms to the discussion of the analysis in Section III-A. We now turn to the nonneutralized IA. Based on the parameters in Table II, the systematic drain capacitance mismatch due to the asymmetrical circuit topology at the current mirror load of the IA was calculated as.fig.9showsthesimulated and theoretical common-mode gain responses for the systematic drain capacitance mismatch only, in comparison with the sum of the common-mode gain responses due to the rest of the capacitance mismatches (i.e., ) for 1% mismatch (i.e., ). As seen from the plot, the theoretical and simulated responses match well, thus confirming the integrity of the analysis. Also indicated from the plot is that the effect of the systematic drain mismatch by far dominates the overall common-mode gain response of the IA, particularly at high frequencies. When compared to a practical case, with say 5% deviation in the capacitance from the optimum neutralization value, we can see in Fig. 9 that the simulated common-mode gain response due to the drain capacitance mismatch can still be kept as low as 60 db even at frequencies beyond the BW of the IA. B. Measured Characteristics For the common-mode measurements, a signal generator (Agilent 33250A) was used to apply a 0.8-V peak-peak sinusoid signal with a dc offset of 1.4 V to the IA inputs, and the residual IA output voltage was monitored on a spectrum analyzer (Agilent E4411B) over the frequency range 100 khz to 10 MHz. Shown in Fig. 10 are the measured common-mode gain responses of the nonneutralized IA and the fixed-capacitor neutralized IA, taken from the average measurements of all 20 chips. The spread of the common-mode gain between all 20 samples featuring the fixed neutralization capacitor was measured to be within 10 db of the average value. Fig. 10. Measured and Monte Carlo simulated average common-mode gain responses for the nonneutralized and fixed-capacitor neutralized IA designs. Let us now consider the high-frequency measured characteristics in the vicinity of the IA s BW at 2 MHz. Without neutralization, the common-mode gain response in Fig. 10 increases at a rate approaching 12 db/octave from about 500 khz up to 5 MHz, beyond which the slope gradually drops and the gain eventually begins to fall as a result of the limited IA BW. On the other hand, with the neutralization capacitor the common-mode gain response stays relatively constant over the frequency range up to 10 MHz. The average common-mode gain improvement at2mhzand10mhzisabout20db and 22 db, respectively. The improvement in the common-mode gain response of the IA at high frequencies due to the fixed neutralization capacitor, is also demonstrated by the postlayout Monte Carlo simulations (taken from the average of 250 runs) shown in Fig. 10. The simulated and measured characteristics follow a very similar trend (the difference is attributed to layout matching limitations). This confirms the effectiveness and robustness against process variation of the drain neutralization technique in suppressing the systematic drain capacitance mismatch. In addition, the fact that the common-mode gain response of the neutralized IA is quite flat against frequencies implies that, in these specific IAdesigns, other random capacitance mismatches are negligible and the systematic drain mismatch is the dominant factor in determining the high-frequency common-mode gain response. This is in line with the theoretical discussion and simulation above. To further support this claim, the effect of the neutralizing capacitance value on the common-mode gain of the IA was examined using the variable capacitor designs. Fig. 11 shows the measured common-mode gain at 2 MHz for a typical IA sample using the nmos variable neutralization capacitor. By varying the dc bias voltage applied to the drain-source terminal of the variable capacitor, the capacitance value could be altered between about 100 ff and 650 ff. As seen in Fig. 11, the common-mode voltage gain of the IA changes as is swept, and a minimum value is reached corresponding to the optimum neutralization capacitance value. All 20 samples exhibited this type of minimum common-mode gain behavior for a specific value (the spread of the value for the minimum is about 20 mv between all 20 samples). Similar

11 WORAPISHET et al.: A CMOS INSTRUMENTATION AMPLIFIER WITH 90-dB CMRR AT 2-MHz 709 TABLE VI SUMMARY OF THE IA MEASURED PERFORMANCE AND SPECIFICATION Fig. 11. Measured common-mode response at 2 MHz for a typical IA sample against the bias voltage applied to the nmos variable neutralization capacitor. By varying the neutralization capacitance value is altered. The minimum value in the common-mode gain corresponds to the optimum neutralization capacitance value. Fig. 12. Measured differential voltage gain and CMRR versus frequency for the fixed-capacitor neutralized IA. The error bars indicate the maximum spread from all chips. behavior was observed for the IA samples using the variable pmos capacitor. The measured differential voltage gain of a typical IA chip sample is shown in Fig. 12. It should be noted that the capacitance neutralization does not affect the differential gain. From the plot, the dc gain is 33.7 db (the deviation between all samples is less than 2%) and the 3 db BW is about 2 MHz. Also showninfig.12isthecmrrofthefixed-capacitor neutralized IA using the measured common-mode gain data in Fig. 10. The CMRR plot in Fig. 12 is representative of typical chip performance. The error bars in the plot indicate the maximum spread from all 20 chips. As seen in the figure, at 2 MHz the average CMRR of the neutralized IA is about 91 db. At this frequency, the lowest and highest CMRR from all samples are 83 db and 101 db, respectively. The measured results of the neutralized IA are summarized in Table VI and these are in good agreement with the specification. VII. CONCLUSIONS We have presented a detailed analysis on the common-mode gain frequency responses of the local feedback IA topology due to mismatches in the transconductance, drain-source conductance and parasitic capacitance parameters in the input stage, from low-to-high frequencies (up to about 10 MHz). The integrity of the analytical expressions for the random mismatches has been verified through simulation. In addition, we have identified that the systematic mismatch at the drain capacitance of the IA with the current mirror load is the major contribution to a low CMRR at high frequencies. To mitigate this effect, we have used capacitive neutralization and demonstrated its effectiveness from the fabricated CMOS IA chip samples, achieving an average CMRR in excess of 90 db up to the circuit s 2 MHz BW. To our knowledge, this represents the best high-frequency CMRR performance ever reported for a CMOS IA with low current consumption. An integrated wideband bioimpedance spectroscopy system using the described neutralized IA is currently being developed. ACKNOWLEDGMENT The authors would like to thank Peter Langlois for his suggestions and help during the testing of the chips. REFERENCES [1] B. D. Miller and R. L. Sample, Instrumentation amplifier IC designed for oxygen sensor interface requirements, IEEE J. Solid-State Circuits, vol. 16, no. 6, pp , Dec [2] V. Schaffer, M. F. Snoeij, M. V. Ivanov, and D. T. Trifonov, A 36 V programmable instrumentation amplifier with sub-20 V offset and a CMRR in excess of 120 db at all gain settings, IEEE J. Solid-State Circuits, vol. 44, no. 7, pp , Jul [3] J.-M. Redouté and M. Steyaert, An instrumentation amplifier input circuit with a high immunity to EMI, in Proc Int. Symp. Electromagn. Compatibility EMC Eur., Hamburg, Germany, Sep. 2008, pp. 1 6.

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S.-Landsiedel, Design of low-voltage MOSFET-only modulators in standard digital CMOS technology, IEEETrans.CircuitsSyst.I,Reg.Papers, vol. 51, no. 1, pp , Jan Apisak Worapishet (M 00 SM 10) received the B.Eng. degree (first-class honors) from King Mongkut s Institute of Technology, Ladkrabang, Bangkok, Thailand, in 1990 and the M.Eng.Sc. degree from the University of New South Wales, Australia, in 1995, both in electrical engineering, andtheph.d.degreeinelectricalengineeringfrom Imperial College, London, U.K., in Since 1990, he has been with Mahanakorn University of Technology, Bangkok, Thailand, where he currently serves as the director of Mahanakorn Microelectronics Research Center (MMRC) and an Associate Professor at the Telecommunication Department. His current research interest includes mixed-signal CMOS analogue integrated circuits, wirelined and wireless CMOS circuits, microwave circuits, and reconfigurable communication systems. Dr. Worapishet is a member of the Analog Signal Processing Technical Committee (ASPTC) of the IEEE Circuit and System Society (CASS), and also a member of the IEICE. Andreas Demosthenous (S 94-M 99-SM 05) was born in Nicosia, Cyprus, in He received the B.Eng. degree in electrical and electronic engineering from the University of Leicester, Leicester, U.K., in 1992, the M.Sc. degree in telecommunications technology from Aston University, Birmingham, U.K., in 1994, and the Ph.D. degree in electronic and electrical engineering from University College London (UCL), London, U.K., in From 1998 to 2000, he held a Postdoctoral Research Fellow position with the Department of Electronic and Electrical Engineering, UCL. In 2000, he was appointed to the academic faculty of the same department, whereheiscurrentlyaprofessorand leads the Analog and Biomedical Electronics Research Group. His main area of research is analog and mixed-signal integrated circuits for biomedical, communication, sensor and signal processing applications. He has numerous collaborations for interdisciplinary research and has published over 150 articles in journals and international conference proceedings. Dr. Demosthenous is a member of the Analog Signal Processing Technical Committee (ASPTC) and the Biomedical Circuits and Systems (BioCAS) Technical Committee of the IEEE Circuits and Systems Society (CASS). He is also a member of the U.K. Engineering and Physical Sciences Research Council (EPSRC) Peer Review College. He is an Associate Editor for the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS II: EXPRESS BRIEFS and an Associate Editor for the IEEE CASS Newsletter. He is on the International Advisory Board for Physiological Measurement, Institute of Physics. In 2006 and 2007, he was an Associate Editor for the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS II: EXPRESS BRIEFS. He is a member of the Technical Programme Committee of various IEEE conferences including ESSCIRC, BioCAS, and ECCTD. Xiao Liu (S 05 M 09) was born in Chengdu, China, in He received the B.Eng. degree in information engineering from Xi an Jiaotong University, China, in 2003, the M.Sc. degree in microelectronics systems design from University of Southampton, U.K., in 2004, and the Ph.D. degree from University College London (UCL), U.K., in From 2006 to 2008, he was a Research Assistant in the Department of Electronic and Electrical Engineering, UCL. Since 2009, he has been a Research Associate in the Analogue and Biomedical Electronics Group, UCL. His main research interests include analog and mixed-signal integrated circuit design for biomedical applications, neuroprostheses and microelectronic sensor design.