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ADS1675 and ADS1675REF VCM Question

John Kasson
Prodigy 90 points
Other Parts Discussed in Thread: ADS1675, THS4503, THS4131, OPA211

We are using the ADS1675 ADC to measure some analog signals and we need to have a very good SNR.

 

What we can obtain is pretty decent by most standards but we need to increase it.

 

We are trouble shooting our board for other issues that may be going on but I have a question about the VCM reference pin on the ADS1675.

 

Right now we have VCM going to a OPA211ID buffer and then to the VOCM pin on a single to differential amplifier (originally a THS4503 but we have since changed it to a THS4131 but with the same results).

 

My question is why do you not bring back VCM to the diff amp on the reference drawings in the ADS1675 datasheet and the ADS1675REF board but instead use a separate source for VOCM on the diff amp?

Thank you

  • 0
    TI__Mastermind 23235 points
    Hello John -
    VCM pin should be (AVDD-AGND)/2 so you should be fine using this for with the differential amplifier. Since the VOCM pin of the differential amplifier is high impedance, you could probably get away with driving the pin directly (without the OPA211); but you would need to determine that based on your requirements.
    The most likely explanation for the reference drawings was to illustrate that you could provide a different common-mode source (external) if desired. Noise on this signal shouldn't be too much of an issue since it would be common-mode noise on the input signal (via the differential buffer amplifier).
    For other suggestions on areas on performance, you might check out the couple of notes on the input clock (CLOCK INPUT (CLK) section (pg 25) of the datasheet).
  • 0 in reply to Greg Hupp
    Prodigy 90 points
    Hello Greg,

    Could you tell me what clock is being used on the ADS1675REF board?

    Thank you,
    John Kasson
  • 0 in reply to John Kasson
    TI__Mastermind 23235 points
    Hello John -
    The clock should be a 32MHz clock from the FPGA (see last paragraph of Section 2.2 of the user guide)