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.