AD7983 input driven by multiplex signals

I want to drive the AD7983 with a multiplexed signal.  I am having a hard time understanding the best time to switch the multiplexor based on the timing diagram in the datasheet.

I would like to switch the multiplexor as soon as possible after the rising edge of the CNV pulse in order to maximize the settling time of the next signal's driving circuitry.  So, I would raise the CNV, wait for the converter to "do its thing", and then switch the mux.  How long must I wait for the converter to "do its thing" before I change the input, without degradation of the conversion.

Looking at the datasheet, I cannot tell if this time is t_CNVH, t_CONV, or some other time.  If it is in fact t_CONV, what causes the 300 to 500 ns span?  And which should I use, 300 or 500?

  • Hello Jake,

    Thanks for your question.

    Once the converter has received the rising edge of CNV, the ADC internal switch will open and the device will then be in hold mode and start converting.Therefore, shortly after the rising edge of the CNV signal, you'll be able to switch your mux channel.wait at least the 10ns, but after that you can safely switch.

    Where is the multiplexer in the signal chain?

    Will you use an ADC driver amplifier between the mux and the ADC input? This would the preferred approach. What amplifier will you use? The amplifier will need the performance noted in the ADC datasheet, but will now also need to be capable of potentially settling a full scale step within the cycle time of the conversion/acquisition (750ns), be mindful of this when you select the driver/amplifier. The ADA4841-1 is one amplifer that should be capable of this.

    Please let me know if you need further advice here or have additional questions.

    Best regards,

    Catherine.

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