Input Capacitance Cancellation Technique
Graduate Category: Engineering and Technology Degree Level: Ph.D. Abstract ID# 88
Self-Calibrated Analog Circuits for Long Acquisitions of Biosignals Chun-hsiang Chang, Li Xu, Kainan Wang, Faculty Advisor: Marvin Onabajo, Analog and Mixed-Signal Integrated Circuit Research Laboratory
Introduction Biopotentials such as electroencephalography (EEG) and electrocardiography (ECG) are conventionally measured using electrodes covered with electrolyte gels or solutions to decrease the contact impedance at the skin interface to values below 10KΩ. But wet-contact measurements cause discomfort and dry out in novel long-term monitoring applications such as in brain-computer interfaces where EEG signals are acquired and analyzed over hours or longer. In general, dry electrodes such as inexpensive Ag/AgCl are better suited for long-term monitoring, but their use is associated with increased contact resistances that can be above 1MΩ. Long-term brain signal monitoring applications would directly benefit from the proposed Self-Calibrated Analog FrontEnd for Long Acquisitions of Biosignals (SCAFELAB) system. The goal of this research is to automatically tune the input impedance of the instrumentation amplifier with on-chip circuits for more accurate acquisitions of biosignals.
Target Applications The proposed technique can enable the use of dry-contact measurements with inexpensive Ag/AgCl electrodes for long-term biosignal monitoring (e.g., EEG, ECG).
Analog front-end circuits & dry-electrode model
Input Capacitance Cancellation Technique • The concept of the proposed input capacitance cancellation technique is based on an integrated negative impedance converter (NIC). • A negative capacitance generation feedback (NCGFB) method was developed, which does not employ extra amplifiers to cancel the input capacitance.
Input Impedance Comparison • This figure shows the impedances at Zinp for the IAs with and without NCGFB in the different process corner cases with Csp= 200 pF.
Zin = -Z when R1 = R2
Basic negative impedance converter
Conclusion • In comparison to an identical IA without NCGFB, the important performances such as CMRR, PSRR, and THD are not significantly Instrumentation amplifier (IA) with direct current feedback and negative affected by the proposed input capacitance capacitance generation feedback (NCGFB) cancellation technique. • The NCGFB does not consume any extra power Simulation Results to boost the impedance from below 20MΩ to • A commensurate IA without NCGFB was designed as reference for comparison. above 500MΩ after the proper adjustment of the digitally programmable capacitors. Performance Power Consumption [μW]
Without NCGFB
With NCGFB
93.6
93.6
Gain [dB]
32.2
32.2
Bandwidth [Hz]
100
100
CMRR @10 Hz [dB]
87.4
87.1
PSRR @10 Hz [dB]
67.2
66.9
THD @10 Hz [dB] for 1 mVpk-pk input
-51.2
-51.1
Output offset voltage [mV]
1.8
1.8
Total input-referred noise voltage [μV] (noise bandwidth: 0.1 - 100 Hz)
2.72
2.72
References [1] C.-H. Chang, M. Onabajo, “Instrumentation amplifier input capacitance cancellation for biopotential and bioimpedance measurements,” submitted to the 2014 IEEE Intl. Midwest Symposium on Circuits and Systems (MWSCAS). [2] L. Xu, J. Feng, Y Ni, and M. Onabajo, “Test Signal Generation for the Calibration of Analog Front-End Circuits in Biopotential Measurement Applications,” submitted to the 2014 IEEE Intl. Midwest Symposium on Circuits and Systems (MWSCAS).
Acknowledgement This work is supported by the National Science Foundation under Award No. 1349692
Self-Calibrated Analog Circuits for Long Acquisitions of Biosignals Chun-hsiang Chang, Li Xu, Kainan Wang, Faculty Advisor: Marvin Onabajo, Analog and Mixed-Signal Integrated Circuit Research Laboratory
Introduction Biopotentials such as electroencephalography (EEG) and electrocardiography (ECG) are conventionally measured using electrodes covered with electrolyte gels or solutions to decrease the contact impedance at the skin interface to values below 10KΩ. But wet-contact measurements cause discomfort and dry out in novel long-term monitoring applications such as in brain-computer interfaces where EEG signals are acquired and analyzed over hours or longer. In general, dry electrodes such as inexpensive Ag/AgCl are better suited for long-term monitoring, but their use is associated with increased contact resistances that can be above 1MΩ. Long-term brain signal monitoring applications would directly benefit from the proposed Self-Calibrated Analog FrontEnd for Long Acquisitions of Biosignals (SCAFELAB) system. The goal of this research is to automatically tune the input impedance of the instrumentation amplifier with on-chip circuits for more accurate acquisitions of biosignals.
Target Applications The proposed technique can enable the use of dry-contact measurements with inexpensive Ag/AgCl electrodes for long-term biosignal monitoring (e.g., EEG, ECG).
Analog front-end circuits & dry-electrode model
Input Capacitance Cancellation Technique • The concept of the proposed input capacitance cancellation technique is based on an integrated negative impedance converter (NIC). • A negative capacitance generation feedback (NCGFB) method was developed, which does not employ extra amplifiers to cancel the input capacitance.
Input Impedance Comparison • This figure shows the impedances at Zinp for the IAs with and without NCGFB in the different process corner cases with Csp= 200 pF.
Zin = -Z when R1 = R2
Basic negative impedance converter
Conclusion • In comparison to an identical IA without NCGFB, the important performances such as CMRR, PSRR, and THD are not significantly Instrumentation amplifier (IA) with direct current feedback and negative affected by the proposed input capacitance capacitance generation feedback (NCGFB) cancellation technique. • The NCGFB does not consume any extra power Simulation Results to boost the impedance from below 20MΩ to • A commensurate IA without NCGFB was designed as reference for comparison. above 500MΩ after the proper adjustment of the digitally programmable capacitors. Performance Power Consumption [μW]
Without NCGFB
With NCGFB
93.6
93.6
Gain [dB]
32.2
32.2
Bandwidth [Hz]
100
100
CMRR @10 Hz [dB]
87.4
87.1
PSRR @10 Hz [dB]
67.2
66.9
THD @10 Hz [dB] for 1 mVpk-pk input
-51.2
-51.1
Output offset voltage [mV]
1.8
1.8
Total input-referred noise voltage [μV] (noise bandwidth: 0.1 - 100 Hz)
2.72
2.72
References [1] C.-H. Chang, M. Onabajo, “Instrumentation amplifier input capacitance cancellation for biopotential and bioimpedance measurements,” submitted to the 2014 IEEE Intl. Midwest Symposium on Circuits and Systems (MWSCAS). [2] L. Xu, J. Feng, Y Ni, and M. Onabajo, “Test Signal Generation for the Calibration of Analog Front-End Circuits in Biopotential Measurement Applications,” submitted to the 2014 IEEE Intl. Midwest Symposium on Circuits and Systems (MWSCAS).
Acknowledgement This work is supported by the National Science Foundation under Award No. 1349692
