Regarding the AD1938 CODEC full-scale input voltage (differential) and reference voltage

Hello UBOTI,

This is a little tricky to "visualize". You have to think of this as two different ways to measure the signal. Either externally using a volt meter differentially between the two pins, and then as two single ended input to two pins. 

Here is a screenshot of the input stage of one of our eval boards that uses the AD1938 codec. The ADCs are differential. 

I drew in the sine waves to show how the two OpAmps will produce the two sine waves out of polarity. 

Each pin will swing the full 2.685Vpp or 1.3425Vp from the 1,5V common mode voltage. If you look at only this signal to this one pin this full scale signal will convert to a signal that is one bit less or -6dB down. You cannot get full scale 0dBFS if you only drive one pin. 

If you are driving the two pins with a signal that is inverted, what happens internally is that we subtract the two inputs to get the final level coming in. I am guessing what is really happening is that we are inverting the signal internally and then adding the two since it is easier to add than subtract but the result is the same. You take a positive going -6dBfs signal and then subtract a negative going -6dBfs signal will give you a 0dBFS level signal. 

This is so hard to describe in writing... The professor in me wants to use a white board!

If you ground one pin and drive the other, you can only get as high as -6dBfs (a 23 bit signal) To get to 0dBFS you have to add two signals together. One from each pin. Since it is a differential input we subtract the two pins. If one signal is inverted from the other then the signal level doubles. If they are the same polarity then they subtract and you get nothing. This would be the common mode noise that you want to reject. 

It works!!

Dave T

Thank you for your reply.

It performs AD conversion on analog input centered around 1.5V at each terminal.

(If com=1.5V=0code and REF is 1.5V, it can handle com±1.5V)
It calculates the conversion results of the positive input terminal and negative input terminal.

Since it does not directly handle differential signals, I understand that REF is fine at 1.5V.

Thank you.