I'm trying to log current readings on a test rig using an ADC11 USB. The circuit is reading the voltage across a 1 ohm shunt resistor in a 24volt dc circuit, the load is between 75 and 200mA, so I'm getting 75 to 200 mV.
I'm using a CA3140E operational amplifier to multiply the input mV by 10, and to stop short circuit to ground. (Differential amplifier mode)
The ADC11 is connected to a laptop and the setup works really well while the laptop is being fed using its internal battery. The trouble is, when I plug the laptop into the mains, the output max.s out to 2.5V (even when switched off)
I think its because the Neutral & Earth path is linked through the test rig and the laptop. I've tryed using an isolating transformer, but I still get max. 2.5V on channel 1
Any ideas what I can do to get the offset back to 0V ?