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ADS8598S: Zero Crossing jumps in measured value

Raman Sridahran
Prodigy 150 points
Part Number: ADS8598S
Other Parts Discussed in Thread: DAC8718

Tool/software:

I am controlling this ADC with a FPGA, and getting a strange glitch when I cross zero. Below is a sine wave input to the ADC, and re-output by a DAC (DAC8718). The DAC appears to work fine, I can output any other codes I generate in the FPGA with no issues.

I am using SPI to communicate, had issues with parallel mode that I am still working through.  But this does not seem - at least on the surface - to be related to poor communication/signal strength.

The output code of the ADC seems to jump up when I cross zero from below; and seems to jump down when I cross zero from above.  Below is the output of the DAC, when a sine wave is read through the ADS8598:

If I don't cross zero, I dont have an issue.  Sampling at 100kHz, schematic below:

I also see another forum topic with this exact issue:

 ADS8345: ADS8345 Zero Crossing Issue 

However I don't see a conclusive result in that thread.

Thanks

  • 0
    Prodigy 150 points

    Interestingly enough, I dont seem to have the issue on ch2 of the ADC.  Could Ch1 just be damaged?

  • 0 in reply to Raman Sridahran
    TI__Expert 3430 points

    Hello Raman,

    Thank you for posting on TI's E2E! Thank you for linking the other E2E post as well.

    Is there a difference in the signal path between channel 1 and channel 2? either in the circuit/schematic or in the input signal. 

    Would you be able to share a schematic showing what the input signal chain looks for the ADC channels?  

    Is channel 1 the only channel that has this issue or does it happen in any other channel?

    Are you using both DOUTA or DOUTB? or just DOUTA? 

    Could you share how the digital signals (SCLK, CS, DOUT, FIRSTDATA) look during this zero crossing glitch? As well as another one that includes CONVST, BUSY, CS, and FIRSTDATA. 

    Are the BUSY and/or FIRSTDATA outputs being used to synchronize the data output? or is the timing for the data out asynchronous to those pins? 

    similar to the other post, do you have a data out log that shows the jump/glitch? 

    Best regards, 

    Yolanda