A High Input Impedance Chopper Stabilized Amplifier Based On Charge Conservation

Abstract: Chopper stabilized amplifiers are popularly used for realizing amplifiers with low offset and for rejecting flicker noise. One of the main limitations of these amplifiers is the low Input Impedance (Zin) produced by the switch capacitor input network. Zin here is resistive due to the switch capacitor action and is inversely proportional to the product of Chopping frequency (Fch) and Input Capacitance (Ci). Since Fch should be greater than the flicker noise corner frequency, this results in a low Zin. When interfacing sensors with high Sensor Output Impedance (Zo), chopper stabilized amplifiers load the sensors resulting in reduced sensitivity. This paper presents a novel input impedance boosting technique - Differential capacitor flipping technique for chopper based Capacitively Coupled Instrumentation Amplifier (CCIA), which prevents discharge and recharge of Ci's in every cycle by reconfiguring the capacitor positions while preserving the chopping operation. This ideally results in a purely capacitive Zin which is independent of Fch. The proposed architecture is used to demonstrate Electrocardiogram (ECG) signal acquisition with dry electrodes that have Zo in the order of a few Mega Ohms. This circuit implemented in TSMC 65 nm CMOS technology node features Zin of 21 GOhms at DC. The circuit has a power consumption of 2.6E(-6)W (2.8E(-6)W including clock generation circuits), with 7.2E(-6)Vrms (1 Hz-150 Hz) of total integrated input referred noise. ~