Differential Lock-in amplifier on shunt resistor

Thread Starter


Joined Mar 3, 2017
Dear AAC community,

I have a small Problem with an measurement circuit and currently on the way with Trouble Shooting. I reduced the Problem to a simple scheme involving a lock-in amplifier (SR830 - Stanford Research Systems) and a shunt resistor. The circuit is given in the attachment. The Signal source is the oscillator of the SR830, which is connected in series via a coaxial cable to a carbon film resistor (10 Ohm) in a open loop configutation. At both terminals of the resistor signal tapping routes coaxial cables to the Inputs of the SR830, which is set to differential mode.

Now I'am wondering to measure a differential voltage in the range of 60 - 80 microvolt, although the circuit is in open Loop?!? There is no dependency on the sensitivity, time constant, floated (10kOhm at Input stage towards ground of SR830) or grounded (10 Ohm) Input Setting and almost no frequency dependency in the desired range of the experiment (which is between 10 and 100 Hz). The voltage scales directly proportional to the Amplitude of the oscillator and the resistance of the shunt (the bigger - the bigger the voltage becomes). The circuit is accomplished by coaxial cables fed to a metallic housing, which comprises the resistor and BNC-ports on a circuit board.

I tried new cables, new Adapters, changed the room/power socket and even borowed a different lock-in model (SR810) to exclude as much possibilities as I could. It always gives the same voltage (except I Change the shunt resistor / oscillator Output voltage).

Does anybody have an idea, what could be the origin of the Problem or a clue of what to test next? I'm grateful for any Kind of idea.

Best regards,