3
\$\begingroup\$

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?

\$\endgroup\$
  • \$\begingroup\$ 24 bit assuming 5V Vcc and 1 bit resolution is ~298nV. This is really challenging! What is the sampling rate? \$\endgroup\$ – Blup1980 Nov 19 '13 at 9:36
  • \$\begingroup\$ Lowest sampling rate possible is 250 samples per second. The software driver is capable of reading 8000 samples per second, the maximum rate of the chip at the full 24 bits of resolution. Yes, it is challenging! Why are you asking about the sampling rate...? \$\endgroup\$ – Adam F Nov 19 '13 at 18:50
  • \$\begingroup\$ Take a peek at your ENOB before you just assume things will work the way you think they will. A tiny bit of amplification may go a long way. \$\endgroup\$ – Scott Seidman Nov 19 '13 at 23:54
  • \$\begingroup\$ ENOB == Effective Number of Bits. The datasheet says at 250 sps, ENOB is 21.30. At 8000 sps, ENOB is 18.62. Both are ok for the application, I think, as long as the board is not introducing a lot more noise. \$\endgroup\$ – Adam F Nov 20 '13 at 0:39
3
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ Yes, you're right - my main reason for wanting the 24-bit resolution is to avoid amplifying the signal very much before it goes into the ADC. (The ADC I'm using does have an on-chip programmable-gain amplifier.) I forgot to mention I that I do have some RC low-pass filtering on the inputs. Good idea on the cables and the active electrodes. I will try to use shielded cables. I will look into active electrodes once I get the main board working. \$\endgroup\$ – Adam F Nov 19 '13 at 18:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.