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ADS7945: Cross talk

ni iv
Prodigy 10 points

Part Number: ADS7945

Hi everyone,

I’m working with an ADS7945 in fully differential mode and switching between channels every 625 ns. At this rate, I’m seeing what looks like crosstalk or residual charge from the previous channel affecting the next conversion. When I slow the channel-switching period to about 1 µs, the issue disappears.

My setup:

  • Inputs are driven by a 50 MHz BW op-amp

  • No input RC network (no series resistor, no Cfilt capacitor)

  • Fully differential configuration

My suspicion is that the ADC’s internal sample/hold capacitor isn’t fully settling when switching this quickly

What can be done in this case ?

  • 0
    TI__Genius 10797 points

    Hi Nick,

    Even though you have a op-amp, it is good practice to have an external capacitor at the input of the ADC. This external capacitor acts as a charge pump, ensuring the ADC internal sample and hold capacitor is properly driven.

    You may take a look at the datasheet (like page 36) for suggestions on sizing this external capacitor. A rule of thumb you may use is C_ext = 20 x C_sample&hold

    Best regards,

    Samiha