Electronic – Amplify difference between two photodiodes

adcamplifierdifferentialoperational-amplifierphotodiode

I have two adjacent photodiodes in photovoltaic mode, with two transimpedance amplifiers. Unfortunately, my MCU (nRF52840) is one meter away, and I expect a fair amount of noise to enter the system. For my application, the only relevant data is the difference in luminosity between these photodiodes.

It seems the right solution would be to perform true differential signaling for each photodiode, but I can't do it in my case for very specific reasons.

By routing each signal next to each other, I hope that most of the noise would affect both signals roughly equally. My goal is to reject common-mode noise, interference, and maximize the dynamic range at the MCU location.

I am considering the following solutions:

  • directly route each signal to the nRF52840 ADC in differential mode
  • use a differential opamp and use the nRF52840 ADC in single-ended mode
  • use a fully differential amplifier to drive the nRF52840 ADC in differential mode

The first solution would be simple, but I am not sure to get enough dynamic range. I am not familiar with fully differential amplifiers and I am not sure about their usage with non single ended signals.

What solution would be the most appropriate? Can you think of a better alternative?

Best Answer

For my application, the only relevant data is the difference in luminosity between these photodiodes.

If both photodiodes are the same, wire them antiparallel to a single TIA. Wiring them antiparallel means that the only current feeding the TIA will be the difference current. Now, the problem of noise is halved. Use a balanced transmission system like this: -

enter image description here

Replace the sensor on the left with a single ended op-amp driver and use balanced cable such as shielded twisted pair. Ground the shield at the receiver end. Clearly you need to use a differential receiver amplifier too. Schematic from here.

The two resistors marked with a blue X represent equal impedance resistors so that a balanced impedance drive to the cable is facilitated and any interference (red lightning symbol) has to affect both inner transmission wires equally. That is what is meant by "balanced" in this context. Of course, with the spare components from the unneeded TIA you can drive the line differentially and increase SNR by another 6 dB.