Ultra Low Power Multistandard G m -C Filter for Biomedical Applications

Transcription

1 Volume-7, Issue-5, September-October 2017 International Journal of Engineering and Management Research Page Number: Ultra Low Power Multistandard G m -C Filter for Biomedical Applications Rangisetti Anitha 1, K.Syam Babu 2 1 Department of Electronics & Communication Engineering, M.V.R College of Engineering and Technology, INDIA 2 Associate Professor, Department of Electronics & Communication Engineering, M.V.R College of Engineering and Technology, INDIA ABSTRACT This paper presents the design of a G-C fourth order butter worth low pass filter for ECG detection. Since the performance of the filter strongly depends on the basic building block of trans conductor cell, a low power, highly linear, pseudo differential trans conductor cell working at 0.5 V in 180nmNWELL CMOS technology is designed. The gain of the designed trans conductor cell is 57 db and the power consumption is 15 nw. The low pass filter has pass band gain 0 db and cut off frequency of 250 Hz is designed. The total power consumption s nw. Keywords ECG signal, EMG, Notch filter, Operational Trans Conductance Amplifier (OTA), Biomedical I. INTRODUCTION Nowadays ultra low frequency filters are using in many applications like hearing aid, wearable breathing detector, neural spike detector and etc [1]. However the biomedical processing and monitoring has to be precise and accurate. With a small noise in the biomedical signal may corrupt the original shape of the input bio medical informative signal. Hence proper filtering has to be needed. Table I shows the different biomedical signals like ECG(Electrocardiogram), EEG(Electroencephalogram), EMG(Electromyographic),etc and its frequency ranges [2]. Even though its frequency ranges are different, every signal can process using same set up is as shown in Fig. 1. Fig 1. Block diagram of biomedical signal processing system As shown in the Table I, all the biomedical signal have almost same magnitude range i.e from 1 µv to 1 mv. But the frequency range is different and the maximum frequency is around 1 khz among all biomedical signals [3]. Hence in order to avoid different individual biomedical signal filtering, one can design a simple multi standard biomedical filter for all signals like ECG, EEG or EMG etc. This paper presents a novel multi standard biomedical filter with ultra low power. A notch filter (50/60Hz) [4], [5] has to be needed to avoid power line interface with information signal as it is easily pick up by electrodes. In this paper a cascaded notch and low pass filters are designed with in a schematic to avoid more area and to avoid extra components and power supply. The designing of ultra low frequency filters is not a simple task because time constant of such filters is around 0.01 s to 1 s range [6], [7]. The time constant of any filter is proportional to RC product but for lower technologies capacitance is limited to 1 pf to 10 pf. In this case capacitance has to be constant so required resistance value should be very high which indicates the effective transconductance (G m ) value should be very low around 1 ns [8], [9]. The G m value can be reduced by choosing bulk driven in place of gate driven inputs because the transconductance of bulk driven mos transistors are less than that of gate driven mos transistors. The effective transconductance can be further reduced by using proposed Operational Trans Conductance Amplifier (OTA) [10], [11], [12]. In this paper Section I describes about introduction. Section II describes about conventional Operational transconductance amplifier design and proposed OTA design. Section III describes about notch filter design and low pass filter design and mathematical analysis of these filters. Section IV describes about simulated results and finally section V describes about conclusion. TABLE I. BIOMEDICAL SIGNAL SPECIFICATIONS Biomedical Amplitude (pp)(mv) Frequency(Hz) Signal ECG EEG EOG EMG k 105 Copyright Vandana Publications. All Rights Reserved.

2 II. OPERATIONAL TRANSCONDUCTANCE AMPLIFIER DESIGN This paper is designed for ultra low power and low frequency (around 100 Hz) applications. Hence the transconductance value G m should be very low. In order to get low G m value, bulk driven OTA schematic is used as shown in Fig. 2. All the transistors used in this OTA are operated in weak inversion (sub threshold) region of operation and while tuning those can be used in saturation region also. Because of wide tuning range and weak inversion region we can attain very low power consumption and bulk driven is chosen to give low Gm value of the order of 50 ns. M 1 and M 2 transistors are two PMOS transistors to which inputs are given to the bulk terminals. M 3 and M 4 are NMOS transistors and biased to dc voltage of Vb 2 to act as constant current source. Where as PMOS transistors M 6 and M 7 are used as negative resistance to improve the gain of the output response. Finally the transistor M 5 and resistors R 1 and R 2 are act as local common mode feedback of OTA. In this paper 0.5 V supply is used hence the input common mode and output common mode voltages has to be adjusted to half of supply voltage i.e 0.25 V. Common mode voltages at outputs can be adjusted by R 1 and R 2. However by using OTA shown in Fig. 2 can be used to tune from 50 ns to 100 ns. Means the minimum value of Gm value with this OTA is only 50 ns. But in order to get 50 Hz notch G m value should be very small. Hence in order to reduce G m value further a proposed transconductor is designed. tunable resistance. NMOS transistors M 5, M 7 and M 6, M 8 pairs act as constant current mirror. The minimum G m value by this proposed OTA is around 0.1 ns. III. MIXED NOTCH - LOW PASS FILTER DESIGN A. Low pass filter design In this paper low pass filter and notch filter can be design with similar schematic with a change of input supply positions. With a single ended supply and with two Gm cells a second order low pass filter can be designed as shown in Fig. 4 and with added extra supply a second order notch filter can be designed with the same schematic as Shown in Fig. 5. The response is given by the following equations. Equation 1 represents the transfer function of second order low pass filter which is shown in the Fig. 4 Since both the Gm cells are identical Gm1 = Gm2 the Eq. 1 can be reduced to Eq. 2 which is in the form of standard second order low pass filter. From the standard second order Eq. 2 the Cutoff frequency can be written as in Eq. 3 similarly the Quality Factor of second order low pass filter is given by Eq. 4 Fig. 2. Conventional G m cell A. Proposed Operational Transconductance Amplifier The bulk driven OTA shown in the Fig. 2 can be tuned only from Gm value of 50 ns to 100 ns. In order to reduce G m value below 1 ns a bulk driven current deviation OTA with local common mode feed back is proposed in this paper which is shown in the Fig. 3. By using this approach G m value can be scale down to 10 to 20 times of original G m value.in this proposed OTA, except M 13, M 14 transistors all other transistors are operating in weak inversion region to consume low power. The input is given to PMOS transistors M 3, M 1 and M 2, M 4 where (W/L) of M 1 is less than that of M 3 similarly M 2 is less than that of M 4. For local common mode feed back PMOS transistors M 13, M 14 are operated in triode region to give a Fig. 4. Second order Lowpass Filter B. Notch filter design As described in the above subsection Fig. 5 shows the schematic of second order notch filter. The equations for the Transfer function, Cutoff frequency (Notch frequency) and Quality Factor are as shown below. 106 Copyright Vandana Publications. All Rights Reserved.

3 Equation 5 represents the transfer function of second order notch filter similarly Eq. 6 gives notch frequency and Eq. 7 gives Quality factor of notch filter. Fig. 3. Proposed Gm cell Fig. 5. Second order Notch Filter C. Mixed Notch-Low pass filter design Since for any biomedical signal from the electrode can be interference with the power line (50/60 Hz) signal, so attenuation of (50/60 Hz) is necessary for all biomedical applications. By using second order notch filter the depth of notch is only around 20 db - 30 db. Fig. 6. Mixed Notch - Lowpass Filter Since this much attenuation is not sufficient for biomedical processing a sixth order notch filter is designed by cascading three second order notch filters shown in Fig. 6. Similarly a fourth order butter worth low pass filter is designed by cascading two similar second order low pass filters.this mixed filter (sixth order notch and fourth order lowpass) is designed by using just ten Gm cells.the Q factors of notch filter and low pass filter has to be same as it is connected in cascade. So in order to give same Q factor of (butter worth) the capacitance values of C 1, C 2, C 3 and C 4 are adjusted with out exceeding 10 pf. IV. RESULTS In this paper with using proposed OTA a sixth order notch filter and fourth order low pass filter are designed and cascaded to give combined notch low pass response. The different performance plots are shown in the following figures. In this paper single ended G m cell are used to design lowpass and notch filters. The G m value for notch filter is fixed to 0.8 ns and the G m value of low pass filter is variable because of this paper is designed for multi standard biomedical applications like ECG, EEG, EMG etc. The Fig. 7 shows the linearity of proposed OTA with G m tuning with bias current. As described earlier the lowpass filter of fourth order is designed with using just four G m cells which will save the power. The fourth order low pass filters can be tuned by using different bias current values. The variable magnitude response lowpass filter of different G m values are shown in the Fig. 8. Similarly the phase response of the multi standard(variable) low pass filter is shown in the Fig. 9. The magnitude response of sixth order G m -C notch filter is shown in the Fig.10 the depth of notch at 50Hz is about -75dB which is more than sufficient for filtering desired signal from power line signal. The phase response of sixth order notch filter is shown in the Fig. 11. The combined frequency response of notch and low pass(cascaded notch and low pass filter) is shown in the Fig. 12. The phase response of this mixed notch and low pass is shown in the Fig. 13. The group delay of mixed notch and low pass filter is shown in the Fig. 14. In order to compare this paper results with the 107 Copyright Vandana Publications. All Rights Reserved.

4 literature survey, two different comparison tables are given individually i.e Table II for notch filter comparison and Table III for low pass filter comparison with the existing technologies. Fig. 10.Magnitude Response of sixth order Notch Filter Fig. 11. Phase Response of Notch Filter Fig. 7. Linearity of Proposed OTA with tuning to different G m values Fig. 12. Magnitude Response of Mixed Notch - Lowpass Filter Fig. 8. Magnitude Response of LPF with different G m values Fig. 9. Phase Response of LPF with different G m values Fig. 13. Phase Response of Mixed Notch - Lowpass Filter 108 Copyright Vandana Publications. All Rights Reserved.

5 REFERENCES Fig. 14. Group Delay of Mixed Notch - Lowpass Filter TABLE II. COMPARISON TABLE FOR NOTCH FILTER WITH LITERATURE SURVEY Parameter Ref[4] Ref[3] Ref [2] Ref[1] This work Vdd(V) Tech. (nm) Zerofreq(Hz) Notchdep.dB Order Structure Gm-C Gm-C opamp opamp Gm-C Power (nw) THD(dB) App ECG ECG ECG ECG,EEG, EMG ECG,EEG, EMG TABLE III. SUMMARY AND COMPARISONS OF LOWPASS FILTER FROM THE LITERATURE SURVEY Parameter Ref[9] Ref[10] Ref[11] Ref[12] This work Vdd(V) Tech.(nm) Bandwidth(Hz) (tunable) Order Structure Gm-C Gm-C Gm-C Gm-C Gm-C Power 10µW 2.5µW 11µW 453nW 96nW THD(dB) DR(dB) V. CONCLUSION This work presents design of OTA for implementing a LPF filter of cut off frequency 250 Hz. In this paper a bulk driven Operational Transconductance Amplifier with local common mode feed back is designed for ultra low power and low frequency applications. For further reduction of cut off frequency, capacitor multiplier is used. The filter designed consumes nw of power at 0.5 V supply in 180 nm NWELL CMOS technology. Simulation results show significant improvement in power, linearity and total harmonic distortion. By using capacitance multiplier, the area used to implement the capacitor is reduced by a factor of about 3. [1] H. A. ALZAHER, N. TASADDUQ AND Y. MAHNASHI, "A HIGHLY LINEAR FULLY INTEGRATED POWERLINE FILTER FOR BIOPOTENTIAL ACQUISITION SYSTEMS," IN IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 7, NO. 5, PP , OCT [2] Li, Haixi, Jinyong Zhang, and Lei Wang. "A fully integrated continuous-time 50-Hz notch filter with center frequency tunability." Engineering in Medicine and Biology Society, EMBC, 2011 Annual International Conference of the IEEE. IEEE, [3] Su, Yu-Cheng, et al. "Wireless ECG detection system with low-power analog front-end circuit and bioprocessing ZigBee firmware." Circuits and Systems (ISCAS), Proceedings of 2010 IEEE International Symposium on. IEEE, [4] Lee, Shuenn-Yuh, and Chih-Jen Cheng. "Systematic design and modeling of a OTA-C filter for portable ECG detection." Biomedical Circuits and Systems, IEEE Transactions on 3.1 (2009): [5] S. Y. Lee and C. J. Cheng, "Systematic Design and Modeling of a OTA-C Filter for Portable ECG Detection," in IEEE Transactions on Biomedical Circuits and Systems, vol. 3, no. 1, pp , Feb [6] Qian, Xinbo, Yong Ping Xu, and Xiaoping Li. "A CMOS continuous-time low-pass notch filter for EEG systems." Analog Integrated Circuits and Signal Processing 44.3 (2005): [7] Ramakrishnan, Shubha, Paul E. Hasler, and Christal Gordon. "Floating gate synapses with spike-time-ependent plasticity." Biomedical Circuits and Systems, IEEE Transactions on 5.3 (2011): [8] Corbishley, Phil, and Esther Rodriguez-Villegas. "A nanopower bandpass filter for detection of an acoustic signal in a wearable breathing detector." Biomedical Circuits and Systems, IEEE Transactions on 1.3 (2007): [9] Solís-Bustos, Sergio, et al. "A 60-dB dynamic-range CMOS sixth-order 2.4-Hz low-pass filter for medical applications." Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions on (2000): [10] Rodriguez-Villegas, Esther, Alberto Yúfera, and Adoracion Rueda. "A 1.25-V micropower Gm-C filter based on FGMOS transistors operating in weak inversion." Solid-State Circuits, IEEE Journal of 39.1 (2004): [11] Qian, Xinbo, Yong Ping Xu, and Xiaoping Li. "A CMOS continuous-time low-pass notch filter for EEG systems." Analog Integrated Circuits and Signal Processing 44.3 (2005): [12] Lee, Shuenn-Yuh, and Chih-Jen Cheng. "Systematic design and modeling of a OTA-C filter for portable ECG detection." Biomedical Circuits and Systems, IEEE Transactions on 3.1 (2009): Copyright Vandana Publications. All Rights Reserved.