nano_voltmeter

A very simple nanovoltmeter with picovolt resolution. The high resolution is possible by using a chopper, auto correlation, a modified delta-sigma analog to digital converter, a 16 bit PWM with passive five pole RC filter configured in a self nulling loop.
All signal processing after the chopper is done in the AC domain with the circuit configured as a nulling amplifier. AC amplifier gain is 10,000,000 and the amplifier is fed into the Arduino ADC. This gives resolution of 100 pV per bit with higher resolution possible because of averaging.
A 16 bit PWM followed by a passive five pole RC filter is used as feedback to the chopper creating a null meter. Because of this feedback, the effective input impedance approaches infinity and gain errors are eliminated. A commodity CMOS op-amp and no precision parts are required to get five digit precision.