I'm using an ac coupled noise matching transformer to measure the voltage noise of a low ohmic sample as a function of bias current. The complete circuit is in an enshielded enclosure. When the transformer primary is fully floating (without R2), the circuit is quite susceptible to external noise. The circuit is shown below
simulate this circuit – Schematic created using CircuitLab
However when a high value resistor to ground (R2) is connected to the primary, the pickup drops significantly.
Why is that?
Best Answer
The transformer inevitably has capacitance between primary and secondary windings and, any common mode signal fluctuations from the "battery powered floating bias current source", will cause a differential signal to be delivered to the single-ended pre-amp via the inter-winding capacitance.
The 1 Mohm resistor provides an alternative shunt path for these common mode signals and thus you see noise reduce. If your pre-amp were differential and balanced you might find that things improve significantly.