So after much work here it is - my week end project to use a HX711 to measure down to nanoamps. Having built it I was amazed how good it is.
I wanted a circuit to measure very low currents to test low power microprocessor circuits.
It has a LCD for local display and can stream data to a PC as a CSV format such that it can be processed in Excel.
Also added autorange so max can be up to 60 mA - measurment uncertainty at low scale 1 nA.
The circuit was built on vero board. Small relay is used to change range by switching in a 1R across the 100R. Values arenot crfitical but you will need to calibrate. Calibration can be achieved for low and high values via serial interface. You must press d on first code upload to set defaults to EEPROM.
The circuit is an ugly bug build. When finished I put the HX711 and circuirty in a metal box for added screening. Although it woked without.
The photos show measuring current from a 33 Mohm resistor sourced from 0.6 volt. Should read 18.2 nA. Photos show -18.5 nA and when reversed 18.05 nA.
The circuit is biased by 1M ohm resistors. The 10k accross E- and E+ is to pull current such that the voltage is stable and 10 uF cap to smooth.
The 100k in series with the 1 Mohm are there because that is what I started with, so just left on the board. I built the code defaults with these values in series. The 100k could be removed.
Code and library is attached, also schematic. The HX711 library is not mine - so achnowledge the author.
Look forward to comments - hope it is of use to someone.