Pseudo-Differential ADC - understanding reference voltages

[Chris42]

Hello,

I'm building a circuit that is supposed to measure light intensity using photodiode.
For transimpedance amplifier I'm using the following circuit with Vbias set to 1.2V.



For digitization I want to use MCP3561 24 bit Pseudo-Differential ADC (https://ww1.microchip.com/downloads/en/DeviceDoc/MCP3561.2.4R-Data-Sheet-DS200006391A.pdf).



When there is not light, amplifier will output biasing voltage so I wanted to set up the ADC to measure difference between output and Vbias.
Ideally Vbias would correspond to code 0 and Vbias + 2V would correspond to the highest code.

My understanding is that I need to connect Vbias to Vin- and output from the amplifier to the Vin+. However I still have some troubles understanding how to connect reference voltages. This ADC has positive reference voltage and negative reference voltage.
My question is what is negative reference voltage used for and how I'm supposed to connect the reference voltages in my case? Do I just connect 2V reference to Ref+ and call it a day or is it more complicated?

[Kleinstein]

The ADC is more like true differential and can have both Ref- and Vin- to move quite a bit. Pseudodifferential would normally be with the negative sides with only a limited range (like +-0.5 V) - normally just used for slightly different grounds.

The ADC does not need an extra negative supply to handle a negative input. So the ADC ref could be fixed like ref- near ground and ref+ at something like +2.5 V to get a range of +-2.5 V  (or less with internal gain).

The circuit as shown may not be suitable to direcly drive the ADC input. The ADC input should be diven with much lower impedance, more like in the 100 Ohms range than the 100 K range.

[MasterT]

Page 85 figure 7-1 Application Example.
ADC doesn't have input buffer, impedance is 260 kOhm at G=1, 102k resistor is no go. Again, look for examples.

[Terry Bites]

Use a good free pd amp design tool. Then an SE to Diff  converter circuit - read: www.analog.com/en/design-center/reference-designs/circuits-from-the-lab/cn0272.html
The differntial photodiode amp has a single bias point for both cathodes, the anodes are a zero volts. Easy.

[Chris42]

ADC doesn't have input buffer, impedance is 260 kOhm at G=1, 102k resistor is no go. Again, look for examples.

That's a good point!

I have updated the circuit (gain was also to low) and now this resistor is 750 ohms.

Use a good free pd amp design tool. Then an SE to Diff  converter circuit - read: www.analog.com/en/design-center/reference-designs/circuits-from-the-lab/cn0272.html
The differntial photodiode amp has a single bias point for both cathodes, the anodes are a zero volts. Easy.

This circuit would be good however it would be nice to get away with just a single rail supply. This circuit is going to be powered from micro USB port so I would need a switching converter to get +-5V rails.