ADD7746 - Testing/Measuring Multiple CDCs Simultaneously?

So looking over the datasheet and the evaluation board instructions, I see how you can test a single CDC.

For my project, I would like to use eight AD7746s. At first, it would seem erroneous to use multiple EVAL-boards.  But I also figured I would reach out to this community so I do not make any assumptions.

My first thought is connecting all the digital inputs (SDA, SCLK, bINT) together into one EVAL board.  But looking at the functional block diagram of the EVAL board, this will not work since the board only has one set of excitation outputs, EXCA and EXCB.

Is there an alternative approach, or is using 8 EVAL boards the worst case scenario?

Would it be cheaper / better if I just do a PCB layout that expands the EVAL board to the number of AD7746s that I need?

Thank you very much.

Allan

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  • Hi Allan,

    I understand your situation better now.  But I see some potential issues with trying to use multiple AD7746's in parallel.  Firstly the AD7746 part has a fixed factory set I2C address so you won't be able to address to multiple parts in parallel.  A work around would be to use a multiplexer switch to switch the I2C lines from the processor to each part as you wanted to read/write to it.  In addition, using multiple AD7746's will mean multiple 32kHz excitations frequencies none of which will be exactly the same (leading to beat frequency interference).  If the sensors or senor connection traces are close together and active at the same time then you could see some coupling of the excitation frequency and this will affect the performance.  By how much I don't know, it would depend on the coupling strength.

    So in summary, you could use the evaluation board as a base and add more parts but you will have to consider the I2C addressing issue and the excitation signal cross coupling between the sensors that will cause interference.

    Hope the above helps.

    Regards,

    Eric.

Reply
  • Hi Allan,

    I understand your situation better now.  But I see some potential issues with trying to use multiple AD7746's in parallel.  Firstly the AD7746 part has a fixed factory set I2C address so you won't be able to address to multiple parts in parallel.  A work around would be to use a multiplexer switch to switch the I2C lines from the processor to each part as you wanted to read/write to it.  In addition, using multiple AD7746's will mean multiple 32kHz excitations frequencies none of which will be exactly the same (leading to beat frequency interference).  If the sensors or senor connection traces are close together and active at the same time then you could see some coupling of the excitation frequency and this will affect the performance.  By how much I don't know, it would depend on the coupling strength.

    So in summary, you could use the evaluation board as a base and add more parts but you will have to consider the I2C addressing issue and the excitation signal cross coupling between the sensors that will cause interference.

    Hope the above helps.

    Regards,

    Eric.

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