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Original question:

ADS8688: ADS8688 2nd Order low pass filter type / Group delay

ADS8688A: Simultaneous Sampling using multiple ADCs in daisy chain

rlc372
Prodigy 50 points
Part Number: ADS8688A
Other Parts Discussed in Thread: ADS8688,

Hi,

I am planning to use multiple ADS8688(A) chips in daisy chain for simultaneous sampling.

The *CS signal seems to control the start of conversion. Is the analog signal value saved to the sample/hold capacitor during the falling edge of *CS?

If that happens, then i guess the variability of the internal oscillator of each chip should not play any role and the multiple ADC arrays will be sampling trully simultaneously. Is that correct?

Thanks in advance!

  • 0
    TI__Mastermind 45215 points

    Hi rlc372,

    The analog input signal will be settled on internal sample-hold capacitor during acquisition(tacq),  /CS acts as a control signal to trigger a conversion on the falling edge. Following graph shows the timing for both acquisition and conversion phases. 

    By sharing /CS signal in multi ADCs, the simultaneous sampling on one channel of each ADC can be achieved. Notice that:

    • The number of devices in daisy-chain configuration should be limited to four or less.
    • The overall throughput of the system is proportionally reduced with the number of devices connected in a daisy-chain configuration.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 50 points

    Hi,

    thanks for your reply. Based on the above description and rechecking the datasheet, this chip is not true simultaneous sampling. This is because the analog value is sampled in the falling edge of the CS*. Simultaneous sampling can only happen on daisy chain with the chan_i in each ADC chip. Is that correct?

  • 0 in reply to rlc372
    TI__Mastermind 45215 points

    Hi rlc372,

    ADS8688A is not a simultaneous sampling ADC, it's not because of /CS signal, it integrates a 9-ch multiplexer and only has one ADC core/one sample-hold circuit. When multiple ADS8688A devices are used in daisy-chain mode and one /CS signal is used for all devices (an example is shown in Figure 92 in datasheet), only one channel on each device is sampled synchronously.

    If you have a simultaneous sampling requirement on all channels of each ADC, ADS8588S or ADS8588H will be the best solution.

    Regards,

    Dale