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Using a single Reference with multiple AD7768-1

Niels
on May 22, 2019

Hello,

I have a design that will use up to 16 (sixteen) AD7768-1 delta-sigma ADC.

Due to limited PCB area, I would like to use a single reference for all AD7768-1.

According to the AD7768-1 data sheet, the AD7768-1 has fully buffered reference inputs REF+ and REF- that provide high impedance input nodes.
From this aspect, I think it should work.

Does the use of a single Reference like LTC6655 with multiple AD7768-1 affect analog performance?

Regards, Niels



(sixteen)
[edited by: Niels at 10:08 AM (GMT 0) on 22 May 2019]
  • FormerMember
    0 FormerMember on May 23, 2019 2:39 PM

    Hi Niels,

    Some things to consider would be:

    • Power. The reference buffers on chip will burn extra power as you will be using 16 of them. Versus one external buffer per 4 or 8 channels for example.
    • Sources of Interference. If you route one reference to 16 different ADCs on the board great care needs to be taken that the reference tracks do not pick up any noise, interference or crosstalk from digital lines, analog inputs, switching power supplies etc.
    • Track length. Ideally the reference should be close to the ADC. Make sure that you have decoupling caps included on the reference inputs of each ADC. 

    Have you considered using the AD7768? There is no sinc3 filter or decimate by 8/decimate by 16 modes on the 8 channel part however. 

    Regards

    Niall

  • 0 on May 24, 2019 7:48 AM in reply to FormerMember

    Hello Niall,

    thank you for your answer.

    My customer wants to have up to 1Msps and up to 24bit resolution. Ideal would be an ADC that delvers 24bit at full 1Msps, but these are "a bit" too expensive. That's why I am planing with multiple AD7768-1.

    My idea was to route the reference as a very thick trace on bottom layer with ground guard traces on both sides, and protected by a ground layer directly below the Bottom-Layer.
    The AD7768-1 ADCs and all analog and digital parts will be placed on the Top layer.
    What capacitors do you suggest for blocking each AD7768-1 REF input?

    Regards, Niels

  • FormerMember
    +1 FormerMember on May 24, 2019 1:58 PM in reply to Niels

    Hi Niels,

    What you have described sounds like it will work, with the internal reference buffer enabled on each channel.

    I suggest to use a 0.1uF cap across each Ref+/Ref- input.

    Regards

    Niall

  • 0 on May 28, 2019 11:58 AM in reply to FormerMember

    Niall,

    thank you for this valuable piece of support!

    Regards, Niels

  • 0 on Nov 25, 2020 9:35 AM

    Hi,

    we have an own Ref+ and Ref- each ADC, but we want to use for all 16-ADCs the same AVDD1&2.
    Do we have to expect a crosstalk between the ADCs?

    Regards, Bernd