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Switching between Multiplexer channels without violating ADC sampling time

Discussion in 'General Electronics Discussion' started by Jimbo bob, Feb 25, 2015.

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  1. Jimbo bob

    Jimbo bob

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    Oct 8, 2014
    Hi all :),

    I recently bought an AD7730LBR Bridge transducer chip so I can use it as an analogue to digital converter for my quarter bridge circuits. I am planning to use four 4051 Single 8-channel Analogue Multiplexers to read the output voltage of around twenty four quarter bridge circuits into my ADC. (I was planning on using two 4067 Multiplexers each of which have 16 channels, but unfortunately they cannot read negative output voltages coming from the bridges.) So, I was concerned with how fast I can switch between the bridge circuits using the Multiplexers without interrupting the sampling time of the ADC. I went through the AD7730LBR datasheet which I have linked, but I could not find anything like sampling frequency or period. I am thinking there must be parameters in there which perhaps indirectly can lead me to the sampling rate, but unfortunately I could not find them.

    I know that many of the dear enthusiasts of the forum may suggest that I should obtain ADC with multiplexers already devised on them which offer more analogue inputs (such as ADS1243 from TI). However, I found out that these chips cost around twice as much as AD7730LBR and I am going to need around four of those. So I thought I would be better off testing my way with external multiplexers.

    I would appreciate any suggestions on finding or calculating the sampling rate. Also If you guys can think of a better way (i.e. less noise inducing way) of inputting twenty four quarter bridge circuits into a high resolution ADC like AD7730LBR, please let me know.

    Many thanks in advanced :D
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You should be able to use the SYNC (datasheeet, page 33, "single shot conversions") pin to reset the ADC within the AD7730 to a defined state when switching channels via the multiplexer. You need to watch out the RDY signal before you read data from the ADC.
     
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  3. Jimbo bob

    Jimbo bob

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    Oct 8, 2014
    I see. I had a read of that section and it is quite interesting. Does that mean the sampling time is variable? I was looking everywhere for a constant value for the sampling time which I would expect to be a multiple of the ADC's clock cycle. Thank you very much for your help :D
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,902
    2,097
    Nov 17, 2011
    When you use "single shot conversions" the sampling period is defined by the interval that your software uses between triggering conversions.
     
    Jimbo bob likes this.
  5. Jimbo bob

    Jimbo bob

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    Oct 8, 2014
    Makes sense. Thanks!!:)
     
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