I am making an 1- 10 kHz AC bridge to measure small changes in capacitance. The bridge is driven by the internal oscillator of a lock-in detector, and null voltage is obtained by adjusting a decade ratio transformer. The other arms consist of two sets of capacitors which share a common plate. Any movement registers an increase in capacitance in one and a corresponding decrease in the other. The common plate is sent to the lock-in for detection. My question is this, I am trying to use some sort of isolation transformer to seperate the ground of the lock in from my bridge and don't know what to use. I have played arround with a variety of transformers (audio etc...) and am not happy with the results. Does anyone have any suggestions.

 

Hi Ribochi:
Could you post a schematic here, for a better understanding of this. It sounds like you are needing to seperate signal paths, not common to the power source, and depending on the isolation needed, it may only be a need for a second power source that may or may not share a common B+. Let us know ok?
Harlan

 

Hi Harlan,

Thanks for the reply, the diagram is basically the general impedance bridge on this website

, where Z1 and Z2 are the inductances of my decade ratio transformer with about 200K impedance, and Z3 and Z4 are my capacitances anywhere from 1 - 5 pF each. The left side of the null detector is the output tap in my ratio transformer which is grounded, and the other side is sent to the Lock-In. I am driving the bridge from the internal oscillator of the lock in and it has a 50 Ohm impedance. The changes in capacitance I am trying to detect are on the order of .000001 pF, which should be achievable according to literature.

Ribochi