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PCM3168 buffered VCOMAD

Pablo Fonovich
Intellectual 765 points
Other Parts Discussed in Thread: PCM3168, PCM4204

Hi...

I previously worked with PCM3168 using the differential buffers shown in PCM4204 datasheet...

Now i want to use the buffers presented in the PCM3168 datasheet, mainly cause of a size problem (the other buffers are to big).

My question is if VCOMAD/DA should be buffered to each input/output buffer, as it's done in the PCM4204 buffer, or can i simply connect VCOM to V+ pin of my OpAmp used in the buffer? i'm not sure if i made my question clear....

Another question... In the PCM3168 datasheet, for differential input, is shown a circuit made only of resistors and capacitors, with no buffer... As i'm trying to do the PCB the smalles possible, this kind of input is tempting me... What problems would use this circuits? i assume impedance copuling would be the problem... am i right?

  • 0
    TI__Genius 9175 points
    The common mode voltage pin VCOM should always be buffered. It does not matter if this goes to the offset voltage pin on a differential input buffer or the V+ on an input buffer.

    VCOM is derived from a resistive divider that is extremely sensitive to loading. If it is not buffered, the potential for the input signal buffer to pull around the voltage on this pin, if directly coupled, is very high. This can create lots of noise and unwanted artifacts. It is also highly recommended to use low leakage electrolytic cap on the VCOM pin before the VCOM buffer to help stabilize the voltage and absorb any voltage fluctuations caused by the proceeding VCOM buffer and input signal buffer.

    As for the passive input, you are correct in that the source impedance will play a big role in the effectiveness. Under certain circumstances the source impedance may also fluctuate with frequency, amplitude, or configurations of the part, causing more issues. An input buffer is always recommended so that the source impedance, filter, and input impedance do not become limiting factors of the performance.

    Regards,
    Matt
  • 0 in reply to Matthew Beardsworth
    Intellectual 765 points

    Matt:

    Thanks for your answer! it came just in time... I was looking at the schematics of the PCM3168 evaluation board, and as i didn't see the VCOMAD buffered, i was going to do without them... but now i will use them for your recommendation...

    So,  i need a simple buffer for each input buffer so i don't overload the pin VCOMAD (as the input current to the operational is very low), or just one buffer with its output connected to all the input buffers V+ pin? the latter option wouldn't overload the opamp?

    And the leackage capacitor... should i use one before each buffer? or just one near the VCOM pin as i do with the power supply pins? If the function is to stabilize the voltage in this pin, i assume only one capacitor near it would do the job... am i right?

    Thanks for your quick answer

    Regards,

    Pablo

  • 0 in reply to Pablo Fonovich
    TI__Genius 9175 points
    Hi Pablo,

    One VCOM unity gain buffer should be fine as the input current into your 5 signal input op-amps V+ legs should be very small. This shouldn’t overload an op-amp buffer if operating correctly.

    And yes put the low leakage electrolytic cap close to the VCOM pin. Only one is necessary as the VCOM buffer will do most of the work.

    Regards,
    Matt