Electronic – Where is this noise coming from? 200 kohm Wheatstone bridge

noisewheatstone-bridge

I have a Wheatstone bridge I want to use to measure a resistive sensor. At the moment, all four arms are fixed resistors, while I get the circuit set up. The noise in the circuit is much larger than I expected, and I can't work out why.

schematic

simulate this circuit – Schematic created using CircuitLab

The AFG is configured to source two sine waves in antiphase on the two outputs. The Stanford is making a DC-coupled differential measurement with an effective noise bandwidth of 0.5 seconds. The two instruments are phase locked using the TTL ref. I'm then looking at the variation of the measurements to work out the noise. I find a noise component which scales with excitation voltage, which I can't identify.

  • The unidentified noise is white from 10 Hz to 1 kHz
  • The amplitude of the noise is about 1 µV/√Hz per volt applied across the bridge
  • When the excitation voltage is zero, I measure a noise consistent with Johnson noise in the bridge.

I have lots of experience of measuring R less than 1 Ω, but 200 kΩ is new to me, so I might be missing the obvious.

Best Answer

I'm betting a component is quantization noise or other distortion on the AFG3000, which has 14bit output. The harmonic distortion is listed as -60dB in your freq range.

You might try using a cleaner sine wave and seeing what you get, or condition the blazes out of the excitation wave you have. In any case, after asking about long cables that may be passing on noise, that would be my first debugging step. Next would be using shielded cables with twisted pairs to drive your bridge, and the next would be using a real bridge amp with sense circuitry.

My money is on the quality of the signal generator, though. I have yet to be pleasantly surprised by the signal quality of an arbitrary waveform generator.