Electronic – What could be the reason for 50Hz noise with isolated input signals

Yesterday I was experimenting with Rigol ds1052e myself and yes you WILL see 50Hz noise with probe without even touching anything. Try to connect ground alligator to the tip of the probe and you will pick up even more noise. This goes away when you unplug probe from BNC. So yes, this is from the probe itself, not necessarily from your circuit. I've seen people suggesting to use coiled piece of wire around ground barrel on the probe, rather than attaching ground alligator. See this link for details http://e2e.ti.com/support/power_management/simple_switcher/w/simple_switcher_wiki/2243.understanding-measuring-and-reducing-output-voltage-ripple.aspx

Also check if your scope has bandpass filter. It might help.

It looks to me like you are using a differential input on your DAQ to measure the voltage across a resistor. It also looks to me like you are exceeding the common mode input signal on your DAQ because of an earth problem. You have one power supply feeding the DAQ and one feeding the current generator. If one power supply has an unearthed output it will likely show a large AC common mode voltage on both wires with respect to earth.

If the unearthed output is the current generator then the voltage across the resistor will be 2V but both sides will be flying up and down together at several volts with respect to earth.

Try connecting the negative output of A1 to the negative output from A2.

You can easily prove it's a common mode problem by trying to measure the voltage at the same end of the resistor. Clearly the voltage should be zero but, due to common mode voltage problems you should see something like the "crazy" signal you currently see.

Here is an idea about common mode noise: -

The common mode noise is the AC on both signals. Between the two signals is a constant offset - this constant offset is what you are trying to measure with your DAQ. I suspect the AC signals on both wires is probably higher than the power supply on your DAQ.