AMP_REF in EVAL-ADAU1467Z Development Board

Hello Eyer1951,

There seems to be some pages of the schematic that were omitted from the user guide. The codec is not even there. The Amp reference is a buffered feed off of the codec CM voltage.

I have attached the PDF of the entire schematic. I think this is the latest revision. I was not the person who designed this but I think this is the latest. It does show the AMP_REF.

PDF

Dave T

Hi David. Thanks for sharing. I've recently obtained the same evaluation board and was wondering the same thing. Except I have another question, forgive my limited knowledge here, but what is the purpose of "AMP_REF" and what is its relationship to "CM_BIAS"? I'm just trying to understand how they have been used here on the evaluation board. Thanks!

Hello,

Amp_ref is used to find the pseudo ground point that is half way between the Op-Amp positive and negative power rails. The OpAmps are being run single ended so you have to setup a reference that is between the two power rails. Often these OpAmps use bi-polar power supplies so there is ground in the middle be default. In this case since it is single ended you have to create it.

Common Mode voltages on codecs are used for similar purposes. The DACs and/or the ADCs are being run single ended so there needs to be a reference that is mid way. The CM voltage comes out onto a pin for two reasons. The first is to filter the voltage with large external caps. The second is for the use of customers that want to DC couple the input or output of a codec. If you DC couple then the external op amps must be referenced to the same voltage reference. If this CM voltage needs to be used then it should be buffered because you must not draw or sink any current from the pin.

Dave T