Differential Difference Amplifier based Parametric Measurement Unit with Digital Calibration

Transcription

1 JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.4, AUGUST, 2018 ISSN(Print) ISSN(Online) Differential Difference Amplifier based Parametric Measurement Unit with Digital Calibration Hyunggun Ma, Gyeong Ho Nam-Goong, Seulkirom Kim, Shin-Il Lim, and Franklin Bien Abstract The proposed PMU with differential difference amplifier (DDA) promises stable operation, accuracy, and reduced power consumption by reducing the number of operational amplifier from five to one. In addition, digital calibration is applied to improve accuracy of forcing ability. The PMU works under various loads from 12.5 Ω to 50 kω. In the forcing voltage mode, the PMU forces a voltage of 0 V to 3.5 V. In the forcing current mode, the PMU forces a current of 0.7 μa to 19 ma. The PMU is fabricated using the Magnachip 0.18-μm BCD technology. The total area of the PMU including the resistor bank is 2 μm 2, and the active area is 0.2 μm 2. Index Terms Automatic Test Equipment (ATE), Parametric Measurement Unit (PMU), Differential Difference Amplifier (DDA) I. INTRODUCTION With the growing interest in semiconductor chips, their applications are getting more diverse and detailed. These growing applications have entailed the demand for a secure reliability for semiconductor chips. Unfortunately, unexpected variations in the electrical performances of the semiconductor chips can occur in the process of fabrication or packaging. Therefore, it is essential to validate every fabricated chip. An inspection procedure can be conducted by a device called automatic test equipment (ATE), which plays a principal role in Manuscript received Oct. 23, 2017; accepted Feb. 26, 2018 Ulsan National Institute of Science and Technology, Ulsan, Korea bien@unist.ac.kr verification tasks. The testing equipment for application processors comprises a driver, comparator, active load, and parametric measurement unit (PMU). Among these circuits, the main component for analyzing electrical characteristic parameters of a test chip is a PMU. It drives voltage or current to the device under test (DUT) to measure the DC characteristic parameters. The primary function of the PMU can be broadly categorized into the following. First, forcing voltage or current to a pin of the DUT, and second, measuring the response from it. Depending on the parameter that is being forced and the response under measurement, the PMU operation is divided into four types, which are force voltage measure current (FVMI), force voltage measure voltage (FVMV), force current measure voltage (FIMV), and force current measure current (FIMI). The most important factor for all the four operations is the accuracy with which the voltage or current is transferred through the PMU. Hence, the PMU uses several amplifiers as a voltage buffer so that it does not interfere with impedance, and uses a negative feedback system with an instrumentation amplifier (IA) to achieve a forcing capability with higher precision. One way to enhance the accuracy of measurement in the negative feedback system is to increase the gain of the amplifier. However, owing to the trade-off relationship between the system accuracy and stability, high gain deteriorates system stability. To design a high gain amplifier without negative effect on the stability, an additional compensation technique is required, which gives rise to additional complexity and high cost. The generally used IA in the PMU has three amplifiers in the feedback loop, which might have a critical effect on the system stability. Also, numerous amplifiers and resistors in

2 JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.4, AUGUST, the feedback system may deliver inaccurate result. In this paper, a PMU structure with only one differential difference amplifier (DDA) and digital calibration is proposed. A primary research was conducted by simulation tool, and confirmed feasibility [1]. A stable operation and an accurate result are achieved by removing numerous amplifiers and resistors in the feedback loop. A guaranteed performance is achieved without additional circuits in feedback loop resulting lower power consumption. In addition, it is possible to force voltage or current with more accuracy by adding digital calibration. II. PMU WITH INSTRUMENTATION AMPLIFIER 1. Conventional PMU Architecture The main function of the PMU is forcing voltage and current to the DUT; however, measuring the voltage or current from the DUT is also an important function. Therefore, forcing accurate voltage or current and a stable operation take priority over measurement capability. Conventionally, the PMU uses operational amplifiers to force voltage and current by simple feedback. Unity gain amplifier and non-inverting amplifiers are suitable candidates for the same application purpose. Fig. 1 shows a conventional PMU structure, which was introduced in [2]. The forcing voltage mode uses a voltage buffer in the feedback loop, and the forcing current mode uses an IA in the feedback loop. The forcing voltage mode is exactly of the same structure as a non-inverting amplifier. The difference is that the feedback voltage is fed back through the voltage buffer. The voltage buffer is used to minimize the impedance variation affected by other circuits by isolating DUT from the measuring pin. Forcing current mode is of a similar structure to a non-inverting amplifier. Since the feedback voltage is a differential voltage, a single input voltage buffer cannot be used. Therefore, an IA is used in the feedback loop rather than a simple voltage buffer. An IA is differential to single ended circuits, which calculate and emit a voltage difference. A 1 and A 2 in Fig. 1 are used to buffer a voltage across Rs and isolate R s from resistors connected to A 3. The output of IA is determined by the ratio of R 1, R 2, R 3, and R 4, which can be expressed by Fig. 1. Conventional PMU structure with instrumentation amplifier. V R = æ R 1+ ö V R - V è ø IA ç FA DUT R1 R3 R3 If R 1 =R 2, and R 3 =R 4, IA works as a unity gain voltage buffer which transforms the differential input to a single ended output. IA makes it possible to force a certain voltage across R s to obtain the desired current. Voltage buffer and IA help to isolate the DUT and R s from other elements, which have the potential to negatively affect accuracy. In addition, they enable negative feedback for a precise forcing capability. 2. Issues in Conventional PMU Among various specifications of the PMU, accuracy and stability are the most important features. Negative feedback is used for improving accuracy. However, stability becomes a concern when negative feedback is used. In the PMU, a circuit with operational amplifiers is located in the feedback loop. As shown in Fig. 1, the forcing current mode in the PMU has three operational amplifiers in the feedback loop. The additional pole and zero from instrumentational amplifier gives possibility of unstable operation. Stability can be achieved by a wellcontrolled pole and zero. The simplest solution is using high speed instrumentational amplifier. However, it is inefficient solution because of increment on power consumption. Offsets in amplifiers reduce accuracy of PMU. Error of resistors in instrumentation amplifier reduces accuracy, (1)

3 440 HYUNGGUN MA et al : DIFFERENTIAL DIFFERENCE AMPLIFIER BASED PARAMETRIC MEASUREMENT UNIT WITH Fig. 2. Symbol of single-ended DDA. (a) Fig. 3. Schematic of DDA. also. Therefore, reducing errors in hardware or removing additional hardware is a solution for increasing accuracy. In addition, accuracy can be improved by stable operated negative feedback. (b) III. PROPOSED PMU WITH DDA 1. Differential Difference Amplifier The DDA is an extended version of the operational amplifier [3-5]. Similar to the operational amplifier, the DDA has positive and negative input, as shown in Fig. 2. The difference between an operational amplifier and the DDA is that the operational amplifier reads single-ended voltage signals, and the DDA reads differential voltage signals. The operational amplifier compares two singleended inputs, and the DDA compares two differential inputs. The output of the DDA is given by ( ) ( ) V = A é ë V -V - V -V o o pp pn np nn where A 0 is the open loop gain of the DDA, (V pp - V pn ) is the positive differential input, and (V np V nn ) is negative differential input. For an incredibly large A 0, (V np V nn ) becomes equal to (V pp V pn ) when negative feedback is applied. Since the concept of the DDA is similar to the operational amplifier, it is possible to construct diverse building blocks with negative feedback. A schematic of DDA is shown in Fig. 3. ù û (2) 2. PMU with DDA (c) Fig. 4. Proposed PMU structure with DDA (a) Total structure, (b) Forcing voltage mode, (c) Forcing current mode. As shown in section 2, the conventional PMU uses an IA to force current. The IA allows to feed a voltage difference between resistors to generate a current. If a circuit that can sense voltage difference exists, then an IA is not needed. A PMU using DDA is shown in Fig. 4(a). A buffer and an instrumentation amplifier are not needed to perform the voltage forcing mode and current forcing mode because the input of the DDA is voltage difference. Therefore, voltage forcing mode and current forcing mode can be performed with a DDA. In the voltage forcing mode, V pp and V np are used as

4 JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.4, AUGUST, input. V pn and V nn are biased to a certain DC voltage; the operation is identical to that of a two input operational amplifier. As shown in Fig. 4(b), voltage mode PMU is similar to a non-inverting amplifier with an operational amplifier. Using non-inverting topology, it is possible to force desired voltage onto the DUT. Since the resistance of the DUT is unknown, an adjustable resistor is used. In the current forcing mode, V pp, V pn, V np, and V nn are used as input. Similar to the voltage forcing mode, noninverting topology is applied. The current forcing mode is shown in Fig. 4(c). If a certain voltage is applied to V pp and V pn, a certain voltage will be applied across the adjustable resistor. The voltage across the adjustable resistor is fed back to DDA. Since the non-inverting topology uses negative feedback and a high gain amplifier, (V np V nn ) will become (V pp V pn ). Then, the voltage across the adjustable resistor will be the same as (V pp V pn ). Using Ohm s law, it is possible to force the desired current to DUT. 3. Digital Calibration In the design of PMU, stability and accuracy are the most important factors. By applying DDA into the PMU, stability ceases to be a concern, because PMU with DDA does not need additional circuits to perform its function. However, DDA is an extraordinary circuit to handle because it has four inputs. The biasing voltage level and signal voltage level affect the gain of DDA. The gain of the amplifier directly influences accuracy. It can be verified through simple feedback system analysis. The system gain of the simple feedback system is calculated as, Y A = (3) X 1 + b A where X is input, Y is output, A is open loop gain, and β is the feedback factor. If we assume βa>>1, the system gain is 1/β. Assume the β is 1, then the output Y is exactly equal to input X. However, if β A is too small to approximate (1+βA) = βa, then output Y will be literally close to input X. Therefore, a high gain is required for an accurate forcing function. Both the force current mode and the force voltage mode have a different biasing approach. In the force current mode, all the inputs are used as input ports, thereby Fig. 5. Flow chart of digital calibration technique for forcing voltage mode. eliminating the need for DC voltage biasing. Since input signal level of the force current mode is very small, the gain of the amplifier does not change. However, in the force voltage mode, two inputs are biased with a certain DC voltage, while two inputs are used as input ports. The DC voltage biasing level and the rail-to-rail input signal level changes the gain of the amplifier. Digital calibration is added to accurately force the desired voltage at the gain error regions of the circuit. The concept of digital calibration involves adjusting the input signal to the desired output value using the back calculation method. Digital calibration is done in unity gain voltage buffer topology, where β is 1. First, digitalto-analog converter (DAC) forces the input signal into PMU. Then the MCU detects the output signal with an analog-to-digital converter (ADC). Since the MCU has an input value and an output value, the gain of the amplifier can be calculated using Eq. (2). With the calculated gain of the amplifier, the input signal value is recalculated to force the desired output value. After digital calibration is done, the calibrated input value for the desired output is stored. Fig. 5 shows the flow chart for digital calibration. In forcing current mode, digital calibration is applied to reduce an effect of the on-resistance in MOS switch. The calibration method for forcing current mode is similar to forcing voltage mode. A 5 kω resistor is chosen as a reference DUT to get calibrated input voltage. First, DAC forces input signal to PMU after setting PMU as forcing current mode. Then MCU detects forced current which is converted to voltage. DUT is used as voltage to current

5 442 HYUNGGUN MA et al : DIFFERENTIAL DIFFERENCE AMPLIFIER BASED PARAMETRIC MEASUREMENT UNIT WITH (a) (a) (b) (b) (c) (c) (d) (d) Fig. 6. Measurement result of forcing voltage mode with various load (a) 12.5 Ω, (b) 500 Ω, (c) 5 kω, (d) 50 kω. Fig. 7. Measurement result of forcing current mode with various load (a) 12.5 Ω, (b) 500 Ω, (c) 5 kω, (d) 50 kω. converter. If the sensed current differ from desired current, MCU increases input signal by LSB. When the sensed current is almost same as desired current, the calibrated input signal is stored. IV. MEASUREMENT RESULTS To measure the performance of the PMU, a resistor back is used as DUT. Resistances of 12.5 Ω, 500 Ω, 5 kω, and 50 kω are used to test the PMU at various loads. AT90PWM316 is used for digital calibration. It has a 10bit DAC and a 10-bit ADC. Fig. 6 shows the results of the forcing voltage mode under various loads. The graph in Fig. 6 is plotted between forcing voltage and voltage forced to DUT. For testing the forcing voltage mode, the PMU is set as shown in Fig. 4(b). Vpn and Vnn are biased with the ground, i.e., 0V. DC voltage is forced to PMU for an input signal. The input DC voltage is varied from 0 V to 3.5 V. Linear output signal is observed. Voltage forced to DUT is the same as the input DC voltage under various loads. Fig. 7 shows the results of the forcing current mode. Different currents corresponding to the respective resistor values are forced to the DUT. For testing the forcing current mode, the PMU is set as shown in Fig. 3(c). Vin in Fig. 7 represents differential value of the input signal. The input signal has 2.5 V as an offset. The offset is added in the input signal in order to operate the DDA in the saturation region. Since DDA only reads differential values of input, the offset is neglected in the forcing

6 JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.4, AUGUST, which is selected for evaluation. The proposed PMU shows stable operation in various loads and a precise forcing ability. Digital calibration is applied to improve accurate forcing ability. The PMU is fabricated using Magnachip 0.18-μm BCD technology, and the active area is 0.2 μm 2. ACKNOWLEDGMENTS This research was supported by the MSIT (Ministry of Science and ICT), Korea, under ITRC (Information Technology Research Center) support program (IITP ) supervised by the IITP (Institute for Information & communications Technology Promotion). Fig. 8. Photograph of proposed PMU with Magnachip 0.18-μm BCD technology. operation. It only affects the operation region of the DDA. The forced DC voltage varies from 25 mv to 300 mv. As shown in Fig. 7, a fabricated PMU can force from 0.7 μa to 19 ma. In spite of digital calibration, errors are observed within the resistance range used for the measurement and the small input voltage range. However, if the range of the resistance is increased and the small input voltage is not used, the error can be extremely reduced. The average error rate of measurement result is 3.3%. Fig. 8 shows an image of a chip of the PMU with the DDA. In addition, the resistor banks and MOS switches are integrated. The PMU is fabricated using Magnachip 0.18-μm BCD technology. The total area of the PMU is 2 μm 2, and the active are is 0.2 μm 2. V. CONCLUSIONS To guarantee accurate and stable operation, PMU with DDA is proposed. The IA in conventional PMU is replaced in to one amplifier, DDA. It enables the PMU to be designed much easier than the conventional PMU with IA. Also, at least one-fifth of power consumption is achieved by removing numerous additional amplifiers. Four loads, 12.5 Ω, 500 Ω, 5 kω, and 50 kω, were used to evaluate the fabricated PMU. In the forcing mode, the PMU can force from 0 V to 3.5 V. In the forcing current mode, the PMU can force 0.7 μa to 19 ma within the load, REFERENCES [1] Kyung-Chan An, Shin-Il Lim, A New Parametric Measurement Unit (PMU) Design with Improved Operating Range in Force Voltage Measure Current (FVMI) Mode, IEIE Summer Conference, pp , Jun [2] E. Collins, I. S. Jung, Y. B. Kim, and K. K. Kim, A Design Approach of a Parametric Measurement Unit on to a 600MHz DCL, 2011 International SoC Design Conference, Jeju, 2011, pp [3] E. Sackinger and W. Guggenbuhl, A versatile building block: The CMOS differential difference amplifier, IEEE J. Solid-State Circuits, vol. SC-22, no. 4, pp , Apr [4] H. Alzaher and M. Ismail, A CMOS fully balanced differential difference amplifier and its applications, IEEE Trans. Circuits Syst. II, Analog Digit. Signal Process., vol. 48, pp , Jun [5] S.-C. Huang, M. Ismail, and S. R. Zarabadi, A wide range differential difference amplifier: A basic block for analog signal processing in MOS technology, IEEE Trans. Circuits Syst.-II, vol. 40, pp , May 1993.

7 444 HYUNGGUN MA et al : DIFFERENTIAL DIFFERENCE AMPLIFIER BASED PARAMETRIC MEASUREMENT UNIT WITH Hyunggun Ma (S 14) received the B.S. degree in electrical engineering from Gyeongsang National University, Jinju, Gyeongsang nam-do, Republic of Korea, in He is currently working on the Combined Master-Doctoral program in Electrical Engineering at the Ulsan National Institute of Science and Technology (UNIST), Ulsan, Republic of Korea. His current research interests are wireless power transfer technologies, especially focused on transceiver ICs using the high-voltage BCDMOS process, readout ICs for flexible touch screen panels and fingerprint touch screen panels, and CMOS RF circuits for biomedical wireless communication. Gyeongho Namgoong received B.S. degree in electronic engineering from Chungnam National University, Daejeon, Korea, in He is currently working on the combined Master-Doctoral program in electrical engineering from the Ulsan National Institute of Science and Technology (UNIST). Ulsan, Korea. His research interests are wireless power transfer technologies, analog circuit design. Seulkirom Kim received his B.S degree in electronic Engineering from Soongsil University, Seoul, Korea in In addition, he received his M.S degree at Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea in His research interests include analog integrated circuit design, communication system and battery management system (BMS) for automotive. Shin-Il Lim received his B.S., M.S. and Ph.D. degrees in electronic engineering from Sogang University, Seoul, Korea, in 1980, 1983, and 1995, respectively. He was with ETRI (Electronics and Telecommunication Research Institute) from 1982 to 1991 as a senior technical staff. He also was with KETI (Korea Electronics Technology Institute) from 1991 to 1995 as a senior engineer. Since 1995, he has been with Seokyeong University, Seoul, Korea as a professor. His research areas are in analog and mixed mode IC design for communication, consumer, biomedical and sensor applications. He has served as the TPC chair of ISOCC 2009 and also was the general chair of ISOCC Franklin Bien (M 03-SM 14) received a B.S. in Electronics Engineering at Yonsei University, Seoul, Korea in 1997, and M.S. and Ph.D. degrees in Electrical and Computer Engineering at Georgia Institute of Technology, Atlanta, GA, USA, in 2000 and 2006, respectively. Dr. Bien is currently an Associate Professor in the School of Electrical and Computer Engineering at the Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea. Prior to joining UNIST in 2009, Prof. Bien was with Staccato Communications, San Diego, CA as a Senior IC Design Engineer working on analog/mixed-signal IC and RF front-end blocks for ultra-wideband (UWB) products such as wireless-usb in 65-nm CMOS technologies. Prior to working at Staccato Communications, he was with Agilent Technologies and Quellan Inc. In an early stage of his career, Prof. Bien s research interests included signal integrity improvement with cross-talk noise cancellation, equalization techniques for 10+Gb/s broadband communication applications, CMOS RF front-end circuits for wireless communications and automotive radar circuits, and adaptive circuits for wireless power transfer (WPT) applications. All of these previous endeavors are migrating in the bio-medical IT research direction to form his current research interests. Multi-target radar technology is moving toward respiratory detecting radar, touch screen panel drive IC & readout IC research is migrating toward biosignature/finger print detectors, and wireless power transfer research is actively being applied to medically implantable devices such as capsule endoscopy, pace makers, cortisol sensors, and in-vivo continuous glucose monitoring systems.