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Analog switch for AD7745

I am designing an application where I need to measure the capacitance of 64 electrodes. The capacitance will be measured individually, one electrode at a time.

I was wondering if you had a recommendation on an analog switch for the AD7745. I plan on switching only the CIN line.

I found the ADG728 which seems appropriate. But I am not sure that the CAPDAC inside the AD7745 can compensate for it.

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  • I am using a ADG1208 on the CINx lines of the ADC. Is there anything to look out for or would the switch introduce measurement error when using the ADC?

    I expect to be able to offset the 10pF capacitance of the switch using capdac of the ADC.

    Also, I plan for single 5V supply,  do I need isolation between the switch and the adc on the power rail?

  • Please refer to figures 25-30 of ADG1208 datasheet.  Depending on the bias level, the switch capacitance will introduce some errors which need to be calibrated out.

  • Hi Nicolas,

    When choosing a ADG12xx series switch key specifications and performance requirements need to be taken into account. The ADG12xx series is designed for low capacitance, low charge injection etc. This low capacitance switch design, results in a higher RON compared to other switches in the ADI portfolio.

    The ADG1208 switch has a high RON of typical 120Ω, this will result in higher dissipation across the switch when a channel is ON and conducting.

    Thanks Vikash for your replies.

    In relation to isolation, I would recommend referring to pages 21 - 23 of the AD7745 datasheet. The datasheet gives an overview of the circuitry connected to the CIN(+)/(-) pins. Vikash is probably in a better position, to answer more specific questions on the relevant information in the datasheet.



  • Thanks.

    According to page 23 of tge datasheet, max serie resistance is 1k, so this should be fine.

    I ordered a test PCB, I'll let you know how it performs.

  • I forgot to update this post, but it works very well. I connected the ADG1208 directly to the AD7746 and I have good performances.