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ADS8353: SAR ADC waveform

jian wang75
Prodigy 60 points
Part Number: ADS8353

   Hi 

ADC input is connecting to the Encoder output (PN is MS0082), the simplified diagram is as below right, offset is 2.5V for encoder output.

as we know, for SAR ADC driving, using an OP-AMP because of its low output impedance.

my question is although this drive circuit is not good, why the waveform at ADC input is as below:

you can see there is a step at middle point (2.5V).

right waveform should be a sine wave with 2.5V offset.

thanks

  • 0
    TI__Guru 66386 points

    Hello Jian,

    Thank you for your post.

    Can you please share a schematic of the ADC connections and the input signal chain?

    Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 60 points

    Hi Ryan

    thanks for your feedback.

    this is used for motor control and this circuit is used for encoder signal feedback.

    the schematic is as below:

    although this circuit has a problem, but i want to understand the ADC behavior under this wrong connection. why there is a step at middle point of sinewave.

    thanks again

  • 0 in reply to jian wang75
    TI__Guru 66386 points

    Hello Jian,

    Thanks for the schematic. 

    This may be related to a settling error at the input pins. One way to test this theory is to reduce the input capacitor. Can you try 1 nF and see if the glitch reduces? Alternatively, reducing the sampling rate will allow for a longer acquisition time and more complete settling.

    For more help on driving SAR ADC inputs, please check out our Precision Labs ADCs training portal!

    Best regards,