# Problem Interfacing with shunt resistor current measurement circuit

Good day all

I am trying to implement current measurement using the shunt resistor method. I have a device that’s powered with 3V and draws 30mA while in normal operation. I’m trying to monitor that current with an Arduino Nano. I’m using a low-side setup like the left hand circuit in the diagram attached and a 1 Ohm resistor to generate a 30mV drop across it.

When I measure the voltage across the resistor with my meter I get the expected 30mV. But as soon as I couple the ground from the circuit to the arduino ground, the measurement drops to 6mV.

I’m not sure whats causing this. Could it be that the arduino ground is somehow offsetting the circuit?
I’ve not been able to find much info on this and am quite stumped at the moment. I may try using a voltage follower to provide isolation.

I appreciate it if anyone can provide an explanation or point out some obvious thing I may be missing.

Thanks

Provide a sketch of how you actually have the components connected - otherwise we'd just be guessing

jackrae:
Provide a sketch of how you actually have the components connected - otherwise we'd just be guessing

I'v added a simple diagram of how it's connected. The arduino is powered via USB

Tokis:
Good day all

I am trying to implement current measurement using the shunt resistor method. I have a device that's powered with 3V and draws 30mA while in normal operation. I'm trying to monitor that current with an Arduino Nano. I'm using a low-side setup like the left hand circuit in the diagram attached and a 1 Ohm resistor to generate a 30mV drop across it.

When I measure the voltage across the resistor with my meter I get the expected 30mV. But as soon as I couple the ground from the circuit to the arduino ground, the measurement drops to 6mV.

I'm not sure whats causing this. Could it be that the arduino ground is somehow offsetting the circuit?
I've not been able to find much info on this and am quite stumped at the moment. I may try using a voltage follower to provide isolation.

You need to measure only the voltage across the shunt. That means using a 4-terminal method - the two
sense wires from the shunt carry no current (ie you cannot use the ground lead from the Arduino as it
carries current, and thus drops voltage unrelated to your measurement).

In this case one way is to run ground from power supply to Arduino, then from Arduino to the shunt. Run a
second wire from the low side of the shunt to another analog input. Measure the difference between the
two analog pins.

The ground is routed this way to ensure the shunt voltages are not below the Arduino ground voltage, since