Just a short post to cover how to measure voltages below 100mV. I recently was trying to use a HP Common Slot PSU to drive a electrolysing unit. Instead of using my own shunt I decided to use Pin 34 IMONITOR to measure the current output.
What i found was that for currents under 5A, I was getting no reading on the ADC. That is because at 11db Attenuation at 12bits, the minimum measurable voltage is 0.1V or 100mV.
Instead of going down the path of using op-amps. I decided to modify my prototype pcb by changing the tradiional voltage divider input into the following.
Datasheets say (from Murata) that the IMONITOR pin puts out 60.15mV/Amp. For the 60.15mV/Amp the swing would be 0 to 3.9V.
So for 10mV of input the ADC would read 1.216V which is well within the linear range of the ADC. At 12bit resolution thats easily doable.
I found with my 750W HP supply, it was 30.15mV/Amp. So for this 62.5A supply, this would mean a swing from 0V to 1.88V.
The ADC would see 1.101V for 10mV of input.
Note that the leftmost resistor can be adjusted to allow a wide range of inputs offset against the 10K/10K/5V divider.
Hope this helps someone out there …
That’s killa