# Differential ADC reference plane - ground or midpoint?

The Microchip (ex-Atmel) ATSAMD10 has a 12-bit SAR ADC with the option to use differential input. I'm generating a virtual ground ($$\V_\mathrm{mid}\$$) at the midpoint (512 mV) of the reference voltage (1.024 V), with the input signal centered around that.

My question is regarding the reference plane for the analog section preceding the ADC. There is a simple 2nd order low-pass Sallen-Key for anti-aliasing, and a placeholder for another RC filter after the Sallen-Key. Should the plane beneath the analog section be ground, or $$\V_\mathrm{mid}\$$? My feeling is that it should be $$\V_\mathrm{mid}\$$ as the filters are all connected to that reference voltage. The only ground connections in that region are for the opamp power.

This is for a two layer board, currently with a ground pour on the bottom layer (green) with some $$\V_\mathrm{mid}\$$ traces through for routing. All the analog inputs are on the bottom of the microcontroller at the top left.

The system is not particularly demanding, but (notional, given the ADC's performance) LSB performance would be nice.

• Source impedance: 18 $$\\Omega\$$
• $$\V_\mathrm{LSB}\$$: 250 $$\\mu V\$$
• $$\f_c\$$: 1.25 kHz
• $$\f_s\$$: 4.096 kHz

• My analog days were dual supply, so I was uncertain about this - makes sense, it's just dumping things to any fixed low impedance. I'll wait a bit before accepting in case there are other thoughts, but otherwise all good :) – awjlogan Jan 26 at 15:17