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Original question:

PCM1865: PCM1865 to OPA1637DGK connection

PCM1865: 10uF input capacitor removal

Ignacio Olazabal
Prodigy 40 points
Part Number: PCM1865
Other Parts Discussed in Thread: OPA1637,

I have a PCM1865 with a differential input from a OPA1637, I've heard that if the Vref (from PCM) is connected to the VCOM (of the OPA) I can remove the 10uF capacitors on the input reducing the manofacturing cost. How true is this? any drawbacks? Would you advice doing it?

  • +1
    TI__Mastermind 31075 points

    Hi Ignacio,

    I believe this advice is coming from the first paragraph in Section 9.1.3 of the OPA1637 datasheet. The idea here is that the analog input pins of the ADC require some bias, typically half the supply voltage. By setting the common mode of the amplifier to be AVDD/2, you set the output of the op amp to exactly match the required bias. This would allow the design to remove the coupling caps since theoretically they are coupled to the same voltage already.

    That's the idea on paper at least. The OPA datasheet uses this advice in general for "precision ADCs" which this device used to belong to. However the PCM1865 datasheet specifically says to leave the coupling caps in place because the device self biases these pins to AVDD/2 on page 28. If there was any mismatch between the bias voltage and the op amp common mode, you would get a DC current that could impact the result of the ADC.

    On paper this idea sounds fine, but since the datasheet argues against it, I wouldn't recommend it as a solution.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 40 points

    First of all thank you for the quick and accurate response. I have a follow up question, you mention AVDD/2, do I need to divide Vref by 2 and then conect to the VCOM? or thats done internally? maybe im missunderstanding.

  • 0 in reply to Ignacio Olazabal
    TI__Mastermind 31075 points

    Hi Ignacio,

    I see your second post, and the answer is the same, so I'll answer on your other post.