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PCM1863: CM voltage injection - Vref

Part Number: PCM1863
Other Parts Discussed in Thread: NE5532

Hello,

I am using a Birt differential circuit in front of the CPM1863 and thus have a good location to inject a CM voltage. This would remove the need to AC couple my signal twice which has benefits.

I observing that the bias voltage on the ADC inputs is the same voltage as VRef. And i was considering supplying Vref as a bias voltage into my Birt Differential receiver; however,  section 9.3.1 of the data sheet recommends against this, but does not consider the use case injecting the CM voltage externally.

Using NE5532's with a max input offset voltage of 5mV, and a series resistance of 100Ω between NE5532 output and ADC input, there is a maximum 50µA push pull between the two devices. Not sure if this is problematic.

The Typical offset voltage of 0.5mV gives potential 5µA current load.

- Adam

  • Hey Adam,

    This is fine to do, just make sure that the reference voltage doesn't have to drive much current. Since you should be interfacing it with the non-inverting op amp input this should be fine (as long as your op amp doesn't have an abnormally low input impedance). 

    Best,

    Zak

  • Sounds good. Do you know what the drive range is? I am using relatively low impedance resistors to keep the noise down. Right now, I have a 2.49k resistor to ground on my non-inverting input (CM injection point).

    Buffering this would negate any benefits as I would be introducing more circuitry, and a servo would be even more costly.

    However, I think I could increase my impedance here. The ADC noise floor is higher than the added noise from the resistors. But it would be good to know how high would be needed to maintain a safe operating area for the ADC.

  • Sorry please disregard that last comment. 

  • Hey Adam,

    I would just remove R92 in this case as it only serves to load the VREF voltage and this will be a much heavier load than is tolerable. I'm guessing this was initially added for bias current cancellation, in which case you would want to connect VREF to the other side of R92 (where GND is right now), but I would not be concerned about the few mV's of error that might arise from bias current, especially if you are keeping the circuit resistance low. If you are disabling the high pass filter of the ADC and trying to make DC measurements then I could understand why you might want to cancel the offset, but since this is an audio part and thus is not trimmed for dc precision, its own offset error will far outweigh that contributed by the bias current.

    Best,

    Zak

  • Exactly. R92 is bias current cancellation. I will probably connect VREF through a 0R to the non-inverting input, and DNP R92. I can always add in resistance for input bias cancellation, but most likely will not. Load on VREF should be minimal.

    Thank you!