Abstract
To realize an ultra-low-power and low-noise instrumentation amplifier (IA) for neural and biopotential signal sensing, we investigate two design techniques. The first technique uses a noise-efficient DC servo loop (DSL), which has been shown to be a high noise contributor. The proposed approach offers several advantages: (i) both the electrode offset and the input offset are rejected, (ii) a large capacitor is not needed in the DSL, (iii) by removing the charge dividing effect, the input-referred noise (IRN) is reduced, (iv) the noise from the DSL is further reduced by the gain of the first stage and by the transconductance ratio, and (v) the proposed DSL allows interfacing with a squeezed-inverter (SQI) stage. The proposed technique reduces the noise from the DSL to 12.5% of the overall noise. The second technique is to optimize noise performance using an SQI stage. Because the SQI stage is biased at a saturation limit of 2VDSAT, the bias current can be increased to reduce noise while maintaining low power consumption. The challenge of handling the mismatch in the SQI stage is addressed using a shared common-mode feedback (CMFB) loop, which achieves a common-mode rejection ratio (CMRR) of 105 dB. Using the proposed technique, a capacitively-coupled chopper instrumentation amplifier (CCIA) was fabricated using a 0.18-µm CMOS process. The measured result of the CCIA shows a relatively low noise density of 88 nV/rtHz and an integrated noise of 1.5 µVrms. These results correspond to a favorable noise efficiency factor (NEF) of 5.9 and a power efficiency factor (PEF) of 11.4.
Highlights
There is a growing interest in wearable, portable, and personal health monitoring.By detecting abnormal health conditions during daily monitoring, this approach provides a new method of preventive healthcare
The results indicate that previous work suffers from high noise contribution from the DC servo loop (DSL) and achieves a relatively low noise-power efficiency
Because f hp created by the DSL depends on the value of pseudo-resistor, we investigate the variability of RDSL1,2
Summary
There is a growing interest in wearable, portable, and personal health monitoring. Concerning the biopotential monitoring applications, sensors and their interfaces providing high-quality signals are of great importance. These sensor devices demand both long operating time and a compact form factor. When a large Chp is used, the result (1) shows that it increases the IRN by charge dividing, causing the DSL to be a high noise contributor; previous studies often neglected this important issue. The proposed approach removes the charge dividing effect and reduces the noise by the transconductance ratio and the open-loop gain This approach solves the problem of interfacing the DSL to the SQI stage, which has a different supply voltage. The result corresponds to a favorable NEF of 5.9 and a PEF of 11.4 by consuming only 0.68 μW, demonstrating a power-efficient low-noise amplifier
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