What are some techniques to lower noise on a mixed-signal analog-to-digital converter board?

Question

I'm building a 24-bit ADC board for Arduino that can sense microvolts (biological signals). What are some good techniques to lower the noise that will negatively affect the ADC resolution?

What I've done already:

• made a 4 layer board, with internal ground plane and digital power plane
• added separate analog and digital power supplies
• kept high-frequency digital signals away from the input lines, and also from the ADC except where they enter the ADC
• most ground connections go to the ground plane using vias
• there is one single ground for both analog and digital ICs
• RC filtering on the input signals

Are there other techniques that would be more effective? Are any of the ones I used counterproductive?

Answer 1

Looks like you are thinking many good thoughts and I can't say any of your points are counterproductive when coupled with careful layout.

As for other techniques, maybe it's worth taking a look at the bigger picture. Here are some ideas for that.

• When using 24 bit resolution (that is a LOT of resolution) for bio signals, you are often really using this for common mode/DC offset where the real signal resolution is only maybe 5-8 bits. There are other ways of achieving both high common mode rejection (instrumentation/differential amplifiers) and DC offset (Right-Leg-Drive) etc.

• Bio signals are often very low frequency, so low-pass filtering at the inputs can reduce the noise pickup.

• The very high impedance nature of typical bio signals results in very low currents. This makes the cable sensitive to noise. If you can shorten the cable, use shielded cables or bring the impedance down (by using an active electrode instead of a passive electrode), that can improve the situation.

As a final note: Think about where you want the return currents to run. That is the key to low voltage/low current design. Make sure you are not sharing the signals return current path with any other currents.