A Low-Noise AC coupled Instrumentation Amplifier for Recording Bio Signals

Transcription

1 Volume 114 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu A Low-Noise AC coupled Instrumentation Amplifier for Recording Bio Signals S.Suhashini 1 and C. Venkata Sudhakar 2 1 Student, M.Tech-VLSI, Department of ECE, SVEC, Tirupati. Mobile No: suhashinishankavaram@gmail.com 2 Assistant Professor, Department of ECE, SVEC,Tirupati. Mobile No: sudhakar.chowdam@gmail.com Abstract The human body is the source of many kinds of signals. These signals are called as bio signals. These signals can be measured by placing an electrode in contact with the human body. These signals will be in the range of few mille volts and also the electrode will induce dc offset to the measured signals. These bio signals should be amplified for further analysis in biomedical applications. The bio signals will amplify by AC coupled instrumentation amplifier. The dc offset also suppressed by the AC coupled instrumentation amplifier. An AC coupled instrumentation amplifier is presented in this paper with low input interfered noise of ~20 and gain of 65 db and CMRR of 142 db with the total power consumption of 104pW. Keywords: AC coupled instrumentation amplifier, CMRR, Bio-signals, Noise, Biomedical applications, DC offset, LTspice software. 1 Introduction The bio signals in human body can be recorded by placing an electrode [2]. Electrode will convert bio signals into electrical signals. The signals amplitude recorded by electrode will be in the range of mv. The bioelectric signals measured from the surface of skin are mostly in the range of 0-2 mv [8]. Bio signals in the human body are electrically weak. This makes the bio signals very susceptible to noise. Noise may come from the devices that are performing the signal acquisition and processing and also from all the other signals that the human body is emitting. The electrode can be a source of distortion that is the electrode introduces offset voltage. Instrumentation amplifier is used to amplify the bio signals recorded by the electrode [1]. Instrumentation amplifier is the first block in analog front end chain that process the bio signals and thus it defines most important specifications like noise level and CMRR of overall systems. Instrumentation amplifier will consume high power in the analog front end and the design should be focused on keeping a good tradeoff between noise and power

2 Instrumentation amplifier is used where high differential gain accuracy, stability must be maintained within a noisy environment and where large common mode signals are present. The applications of instrumentation amplifier are medical instrumentation, data acquisition system, audio application involving low amplitude audio signals in noisy environment to improve signal to noise ratio.in this paper we present an ac-coupled instrumentation amplifier for recording bio signals. The ac-coupled instrumentation amplifier will suppress the dc offset and it will increase the gain and CMRR of the signal. 2 Existing system The output signal of the transducer is given as the input to instrumentation amplifier. The transducer outputs are of very low level signals. So, it is necessary to amplify the level of the signal before the next stage while rejecting noise and the interface. Amplifiers must have high common mode rejection ratio (CMRR) for the rejection of noise. An instrumentation amplifier is usually employed to amplify low level signals, rejecting noise and interference signals. Fig. 1. Instrumentation amplifier The most commonly used instrumentation amplifier consists of three opamps as shown in Fig. 1. The op-amps at input stage are amplifiers are noninverting amplifiers. The output stage op-amp is differential amplifier. By placing the Rgain between the input stage amplifier will eliminate the need of impedance matching. The output stage of the instrumentation amplifier is a differential amplifier, whose output Vout is the amplified difference of the input signals applied to its input terminals. If the outputs of input op-amps are Vo1 and Vo2 then the output of the differential amplifier is given by Eq. 1. The overall voltage gain of the instrumentation amplifier is given by Eq. 2. (1) In standard CMOS technology by using laser trimming method we can achieve perfect impedance matching. But it is very expensive. This topology is easy to implement but not very effective in low noise and low power applications. The 3 stage op-amp also consumes high power. High gain at input stage of (2) 2 330

3 instrumentation amplifier is difficult to achieve due to the electrode offset values. These dc values can suppress the gain of the instrumentation amplifier. Due to this Ac coupled instrumentation amplifier is used to suppress the dc offset and to increase gain and CMRR. 3 Proposed system The processing of low level ac signals in the presence of both common mode noise and differential dc voltage is needed in many applications. In such conditions, AC coupling to instrumentation and differential amplifiers is needed to extract the ac signals by rejecting both common mode noise and dc offset[4]. This situation typically occurs in bioelectric signal acquisition, in which metallic electrode polarization produces a large dc voltage, ranging from 0.15V, which adds to low level bio-logical signals. AC coupling allows only AC signals to pass through a connection. AC coupling removes the DC offset by making use of DC-blocking capacitor in series with the signal. The DC offset introduced by the electrode [7] is suppressed by AC coupled instrumentation amplifier. This can be achieved by placing passive elements like resistors and capacitors. The resistor R1 and capacitor C1 will set the high pass cut-off frequency at the frequency of. All resistors are implemented with diode connected PMOS devices. These resistors are predefined pseudo resistors that are biased in sub threshold region to achieve large resistance values by keeping silicon area small. This design can eliminate the DC offset[5].the AC coupled instrumentation amplifier design is shown in Fig. 2. The Transfer function of the AC coupled instrumentation amplifier is given by Eq. 3. The first stage input amplifiers A1 and A2 are designed by Operational transconductance amplifier. The second stage amplifier A3 is designed by miller compensated amplifier. (3) Fig. 2. AC coupled instrumentation amplifier 3 331

4 A. Input Amplifiers The pre amplifiers A1 and A2 are identically fully balanced operational transconductance amplifiers (OTA) [6]. The function of OTA is to convert an input voltage into an output current. The output current of an OTA is proportional to the difference between the input voltages. OTA provide high input and output impedance. High input impedance allows maximum transfer of the source voltage to the input of the OTA. Maximum transfer of the output current to the load occurs when the output resistance is high.the balanced OTA is designed as shown in Fig.3. In the above OTA design transistors M1=M2, M3=M4, M5=M6, and M7=M8.The ratios of transistors are given below in the tabulated below. MOSFET M1,M2 M3,M4 M5,M6 M7,M8 B. Second stage Amplifier The second stage amplifier is a differential amplifier. The second stage amplifier function is to subtract the output voltages of input amplifiers. This operation increases the CMRR of the instrumentation amplifier. The miller compensated op-amp is selected for second stage amplifier A3. Miller compensated op-amp have high open loop DC gain. Fig. 3.First stage Balanced OTA Fig.4.Second stage Miller compensated amplifier 4 Simulated results The instrumentation amplifier has been simulated using LTspice XVII tool. A. Existing system results a) Common mode input: Instrumentation amplifier both input terminals V1 and V2 are connected to the signal with 100mv amplitude as shown in Fig. 5. The 4 332

5 input signal is 100mv in amplitude and the common mode output signal Vcm amplitude is 42 V shown in Fig. 7. b) Differential mode input: Instrumentation amplifier input V1 is connected to a sinusoidal signal with 100mv amplitude and V2 is connected to the ground as shown in Fig. 6. The output signal Vdf amplitude is 2V shown in Fig. 7.The input signal is amplified to 2V. Fig. 5.Common mode input for the IA Fig.6.Differential mode input for IA The transient analysis output is shown in Fig. 7. The power consumption of instrumentation amplifier is 90 W shown in Fig. 7 c) AC analysis: For AC analysis the V1 and V2 are connected to a sinusoidal signal with 1V amplitude. The AC analysis of IA is shown in Fig. 8. At 1Khz the values of,, CMRR are Common mode gain ( Differential mode gain ( ) = db ) = db CMRR = = db. Fig. 7.Output waves forms of transient analysis Fig. 8. AC analysis of IA 5 333

6 d) Noise analysis: The noise analysis of instrumentation amplifier is shown in Fig. 9.The input referred noise of instrumentation amplifier is B. Proposed system result Fig. 9. Noise analysis of instrumentation amplifier The Operational transconductance amplifier (OTA) is chosen for input stage amplifiers. The OTA design is shown in Fig. 10. The miller compensated amplifier is chosen for second stage amplifiers. The miller compensated amplifier design shown in Fig. 11. AC coupled instrumentation amplifier implementation using LTspice is shown in Fig. 12 and Fig.13. Fig.10. OTA design Fig.11.Miller compensated amplifier design a) AC analysis: For AC analysis the Input amplifiers are connected to Sinusoidal signal with 1V amplitude. AC analysis of AC coupled IA is shown in Fig. 14. At 1Khz the values of,, CMRR are Common mode gain ( Differential mode gain ( ) = db ) =64.97 db CMRR = = dB

7 Fig.12. Common mode input for Ac coupled IA Fig.13.Differential mode input for AC coupled IA b) Noise analysis: The noise analysis of instrumentation amplifier is shown in Fig. 15. Fig.14.AC analysis of AC coupled IA Fig.15. Noise analysis of AC coupled IA The input referred noise of AC coupled instrumentation amplifier is. The power consumption of AC coupled instrumentation amplifier is pw. 5 Conclusion A low noise AC coupled instrumentation amplifier is designed and simulated using LTspice software. The AC coupled instrumentation amplifier consumes less power of 104pW. It has low input interfered noise of ~20 (less than 1 High CMRR of 142 db, and high gain of 65 db. These results are compared with the instrumentation amplifier results are shown in comparison table. The AC coupled instrumentation amplifier has high CMRR, less power consumption, less input interfered noise. Table 1: Comparison of results Existing System Proposed System Gain 54dB 65dB CMRR 81 db 142dB Input interfered noise ~81 ~20 Power consumption pW 7 335

8 References [1] E.Paraskevopoulou and Amir Eftekhar and NishanthKulasekeram.," A lownoise instrumentation amplifier with DC suppression for recording ENG signals, "IEEE Trans. Biomed. Eng., pp , [2] X. Navarro et al., A critical review of interfaces with the peripheral nervous system for the control of neuroprostheses and hybrid bionic systems, J. Peripher. Nerve. Syst., vol. 10, pp , [3] H. Wark et al., A new high-density (25 electrodes/mm2) penetrating microelectrode array for recording and stimulating sub-millimetre neuroanatomical structures, J. Neural Eng., vol. 10, no. 4, p ,2013. [4] L. Andreasen and J. J. Struijk, Signal strength versus cuff length in nerve cuff electrode recordings, IEEE Trans. Biomed. Eng., vol. 49, no. 9, pp , [5] R. Rieger et al., Very low-noise eng amplifier system using cmos technology, IEEE Trans. Neural Sys. andreh. Eng., vol. 14, no. 4, pp , [6] Twinkle Patel, KishanRaikar, SharanHiremath, Prof. SnehaMeti, Design of Balanced Operational Transconductance Amplifier (OTA), International Journal of Emerging Technology in Computer Science & Electronics (IJETCSE) ISSN: Volume 14 Issue 2 APRIL [7] F. Rodrigues et al., Design, fabrication and modeling of a cuff electrode for peripheral nerve stimulation, IEEE ENBENG, pp. 1 4, [8] A. Uranga, X. Navarro, and N. Barniol, Integrated CMOS amplifier for ENG signal recording, IEEE Trans. Biomed. Eng., vol. 51, no. 12, pp , Brief profile about Ms. S. Suhashini S.Suhashini Received B.Tech degree in Electronics and Communication Engineering from APIIIT, Idupulapaya in the year 2015 and M.Tech in VLSI from Sri Vidyanikethan Engineering College, Tirupati in the year Brief Profile about Mr. C. Venkata Sudhakar C Venkata Sudhakar received the B.Tech Degree in Electronic Instrumentation and Control Engineering from S.V.University, Tirupati, Andhra Pradesh, India in 2006, and the M.Tech. Degree in Digital systems and Computer Electronics from J.N.T.U.H. Kukatpally, Hyderabad, Telangana India in Present working as Assistant Professor in the Department of ECE in Sree Vidyanikethan Engineering College and Pursuing the Ph.D. (Part Time) degree in Electronics and Communication Engineering at S.V.University Tirupati

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