DC offsets varies from room to room

[miked3]

Hi,
I'm using a Rigol PA1011 to drive a small voice coil with about 1W peak at 4Volts.
I switch labs from time to time and will see a DC offset appear on the amplifier output. The entirety of the setup is identical, just different rooms. It usually about 1V, but can be as high as 10V. In some rooms there is none at all.
I'm guessing the amplifier is being affected by the quality of the ground in the room? I've used other amplifiers as comparisons, namely a Krohn-Hite 7500 and an AE Techron 7224, and they show no DC offset.

So here are the questions.
1. What do you think is the cause of this?
2. If I'm shopping for a new amplifier, what spec should I look at to determine if the amplifier will be resistant to whatever is causing this?

Thanks a lot,
Mike

 

[FvM]

Sounds curious. Normally we should assume that the amplifier is either defective, overloaded or otherwise operated incorrectly. What's the coil impedance and signal level?
Did you check the input and output waveform with an oscilloscope?

I presume the DC offset option is disabled.

 

[miked3]

I've attached a plot of the outputs as measured with an NI 4431 daq.

 

[chuckey]

The wave form suggests its related to 50 HZ mains.
Frank

 

[miked3]

The wave form suggests its related to 50 HZ mains.
Frank

Sorry, I should have been clear that the bumps at ~0.01s and 0.03s are the waveform I'm inputting myself.

 

[dick_freebird]

Rectification of ambient EMI is one likely culprit. Could be the 50Hz
or could be the omnipresent RF crud from all the test gear (or in
some other case, the absence of).

Especially in circuitry that has a ground-centered signal of interest
and ground-returned ESD clamps, significant charge pumping can
occur. Only with clamps reverse biased to beyond the incoming
crest voltage can charge pumping be neglected.