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derekoc · Newbie Joined: 07 Mar 2008 Posts: 1 Location: UK

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 ?

PeterF · Posted: Mon Mar 17, 2008 3:39 pm Post subject:

Hi Derek,
It is difficult to be sure exactly what is causing your problem without being able to view and measure voltages but I will go through some possibilities.
Your amplifier is probably being overloaded because the input lines are being pulled out of the common mode voltage range. There are two ways this can happen when you plug-in a laptop power supply, firstly by direct leakage through to earth and secondly by noise injected by the switched mode power supply. In either case there is a voltage offset generated between the laptop/scope combination and the circuit being measured.
Things you could try are:-
1. Earthing the laptop chassis (any external metal part)
2. Earthing the USB shield
3. Connecting the Laptop chassis to the circuit under test 0V
4. Adding Ferrite rings to laptop PSU lead
5. Any combination of the above!
Best of luck!
PeterF.

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