SparkFun Forums

[Khumarahn]

Hi.

I have a computer with arduino plugged into USB port. I also have two contacts with a small voltage between them (0-50mV), and I want to measure it accurately. The problem is, I don't know the common mode voltage, and it may be like 400V.

I would use an instrumentation amplifier, but the common mode voltage can be too high... I would use isolation amplifiers, but those I found have at least 1mV offset voltage.

Any ideas how to measure such small voltages?

[lyndon]

Simplest way is probably a second, battery powered, Arduino to measure the voltage and communicate it to the first Arduino over an opto-isolated link.

Or one of these.

[Khumarahn]

lyndon, thank you much

I found that there exist isolated dc-to-dc converters, and I could use on of these instead of a battery... And they can even convert a single supply into a dual one.

And I will probably use a dedicated ADC with an instrumentation amplifier instead of the arduino on the isolated side...

I don't understand however, if there will be any significant noise, if common voltage is AC.

[Valen]

Does it need to be measured continously, or can it be done intermittantly?

If the latter. Use 2 pairs of 2 relays (sort of a H-bridge configuration) to alternately connect a capacitor between the 2 contacts and the measuring inputs of the Arduino. 2 relays could connect the capacitor to the 2 contacts while the other relays to the arduino side are disconnected. Once the capacitor is charged to the small voltage difference all relays need to be disconnected from it. The capacitor would keep it's charge and voltage difference. Then the relays on the arduino's side would take it over and connect the capacitor to the Arduino analog input and ground for measuring. But you might also want an opamp circuit in between to buffer and amplifiy the low voltage of the capacitor.