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ADS8556: turning internal buffers off to reduce the error

Viktorija Cecil
Mastermind 18491 points
Part Number: ADS8556

Team, 

My customer is working with their hardware setup for the ADS8556, and was wondering about the “Reference Buffer Enable” REFBUF_EN (pin 14):

When would be an example of disabling the internal reference buffers.

The question we are wondering is, if by turning the internal buffers OFF, we can reduce the error by applying the reference voltage directly to each SAR’s Reference pins (is the below a recommended configuration?):

 

Thanks

Viktorija

  • 0
    TI__Guru* 97655 points

    Viktorija,

    I don't have any specific data on the error of the reference from the buffer. However, with the reference driving four inputs, I think the buffer would be very important. The reference input current can be large and variable with input signal. With the combined load of four ADCs, I think just the load regulation of the reference would be a significant error. 

    Just doing a quick look, I haven't seen any information about the characteristics of this device with the buffer off. I would guess that the buffer is really required for good performance.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    The question really comes to how would we analyze which is better.  The slight offset errors introduced by the buffer OR the load regulation profile of the Reference Voltage.  With the buffer underspecified it is hard for us to make the best design trade off around it.

    The other question that comes up then is.  When/Why would you disable the reference buffers?  Is their an example applications where one would do this?

  • 0 in reply to Peter Van Calligan
    TI__Guru* 97655 points

    Peter,


    Sorry I didn't respond, but I was out of the office at the end of last week.

    I haven't been able to find characterization data of the device with the reference buffers disabled. My guess is that the primary use case is with the reference buffers enabled, and I haven't found any instances or use cases where any further characterization was done.

    I'll check with some of the other applications engineers, but I'm not sure there's much more on using the device without the reference buffers.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 97655 points

    All,

    I did check with one of the other apps engineers that covered this device, but forgot to post back. He said that without the buffer there might be some benefit to the gain error (because the buffer would add some amount of offset and drift). However he's never tested it and he's never seen an application that uses the device without the reference buffer.

    In the end, the device isn't a super precision ADC. If you used the buffer with a good reference, there might be some degradation, but we don't have any specifications on how much. There would almost certainly be some contribution of error depending on the drive capability of the reference. For the internal reference, I don't have any data for that.


    Joseph Wu