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MUX36D04: directly use MUX36D04 to measure Li-ion battery voltage

Jiandong Liu
Intellectual 1801 points
Part Number: MUX36D04
Other Parts Discussed in Thread: TIPD151, OPA192

Hi expert,

In TI reference design TIPD151, there are OPA192 working as buffer before MUX36D04.

I'm wondering could I remove OPA192 and directly use MUX36D04 to measure Li-ion battery voltage? Will the measurement accuracy be affected? And what's the reason? Thanks.

Regards,

Jiandong

  • +1
    TI__Genius 10974 points

    Hi Jiandong,

    The op-amp buffer is placed there to ensure any source resistance (or any other path resistance between the signal and the mux) is mitigated by the high input impedance of the buffer and the mux sees the most accurate signal it can as this is being used in a differential setup. By removing these buffers, you will face reduced accuracy as you have to account for the voltage drop of the source resistance/path resistance without these high impedance inputs.

    Thanks!

    Bryan

  • 0 in reply to Bryan Marshall
    TI__Intellectual 1801 points

    Hi Bryan,

    Clear explanation. Many thanks!

    Regards.

    Jiandong

  • 0 in reply to Bryan Marshall
    TI__Intellectual 1801 points

    Hi Bryan,

    Sorry for double reply. 

    In my understanding, if the source resistance is very small(just like below figure), a buffer behind MUX can already make sure very high measurement accuracy. So the buffer between source and MUX is not necessary. Do you think is it right?

      Looking forward to your reply. Thanks!

     Jiandong

  • 0 in reply to Jiandong Liu
    TI__Genius 10974 points

    Hi Jiandong,

    If the source resistance is that small, it probably will not affect the reading too much at all. Something that needs to be considered by having that extra buffer is the smaller voltage drop across the source resistance. For example, with the buffer removed, the total current going through that path will be equivalent to the sum of the ION of the MUX and the input bias current of the amplifier (8pA for the mux and 5pA for the op amp, so 13pA total). As opposed to with the buffer (only 5pA load seen by the source impedance, so a lower drop overall). In this case with such a small source impedance, the impact will be minimal. However, with larger impedances, the accuracy could be impacted to a more significant degree.

    So in this instance, it probably is not necessary to have that first stage buffer.

    Thanks!
    Bryan