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ADS8638: 91Hz Spurious in ADS8638

Derek Wang
Intellectual 615 points
Part Number: ADS8638

Hi, 

My customer design ADS8638 as below cirucuit, and the power source is described in below figure.

Their application is vibration.

The input signal is 3.3V from power supply.

Sampling Frequency is 25KHz.

And they found that there is a spurious appear at 91Hz if they use ADS8638.

At the same conditions, there is no spurious if they use ADI AD4000

Test results are shown as below.

Could you give us some advice to avoid the 91Hz spurious ?

TI ADS8638

ADI AD4000

  • 0
    TI__Mastermind 24940 points

    Hello,
    We are looking into this, and will get back to you soon.
    In the mean time, I took a quick look at the schematic you provided.
    The REF pin capacitor needs to be larger, 10uF, as well as the HVDD decoupling cap, to 1uF. Also it seems there is no decoupling capacitors on AVDD, only on DVDD, capacitor needs to be added to AVDD, same as on DVDD.  Insufficient decoupling capacitance may allow noise noise through.  


    Regards,
    Cynthia

  • 0 in reply to Cynthia
    TI__Mastermind 24940 points
    After further review, would you provide the following:
    -What is driving circuit (op amps) to the ADC inputs?
    -Are the 4700pF capacitors populated? This is a large size for capacitors and would need to be analyzed in combination of the driving circuit to make sure the input signal is adequate.
    - You mentioned the input signal is the power supply, is this a DC value? is this for test purposes or is this circuit meant to sample the power supply in the application ?

    In the schematic, it shows that the reference is driven by a step down converter, this is highly discouraged as DC DC converters as they can introduce harmonics into the circuit.
    As for the AD4000 device, this is a 16 bit device, why are they comparing a 16 bit device to a 12 bit device? When they use the AD4000, are they running it under any special modes, such as high Z mode,?

    Regards
    Cynthia
  • 0 in reply to Cynthia
    TI__Mastermind 24940 points
    I am hoping that you have not responded because the customer was able to find the issue and fix it.
    If not, please let me know the status. The information in the previous post would also be helpful if debugging is still necessary.
    Regards, Cynthia