What is a AC ground ?

[MogaRaghu]

I recently configured a PCI 6221 for Differential Input and fed the input with the signal from a digital panel meter's retransmission output ( +/- 10V level ) The DPM was powered by the mains and did not have the third earth pin - just the phase and neutral terminals.

Thus  the DC side of the DPM electronics was powered by a step down transformer / diode bridge / 7812 regulator. As the DC ground was not linked to the mains earth , the voltage output could be termed as "floating source " ?

But when I connected this output to the differential input, I was surprised to  find it working normally even without any  bias resistors. Somebody  said  this  was working because of the  AC ground on account of the step down transformer.

I am not sure I followed - can some one clarify this point a bit more ??

Thanks

Raghunathan

[AnalogKid2DigitalMan]

Do you know the output impedance of the DPM? Wondering if it is quite low and that's why differential is working.

I do not agree with the logic regarding AC gound and the transformer.

For kicks, you could ohm out the DPM outputs, each one individually with respect to AC ground.