AMC1311: What's the reasonable range of VCM_ADC in the application AMC1311EVM

Sorry, now I understand.

But I still don't know what's the advantage of VCM_ADC in this application. If VCM_ADC is connected to GND, I can save the voltage following circuit, and Vout=VoutP-VoutN directly. Is there any disadvantage if I used that?

THanks a lot!

JIN JIAN

Hi Jin,

you can handle only one signal polarity then.

Kai

Hi Jin,

The point of VCM_ADC was to allow you to connect up an external stand-alone ADC, or the ADC that you might find in a typical micro controller.  In the first case, you might have an ADC that allows a 0-5V input range.  You could add a little gain to the differential to single ended buffer and adjust the common mode to perhaps 2.5V so that you take advantage of the full scale range of the device.  Same is true for the ADC in a controller, where its mid point voltage might be 1.65V or 1.25V.  If you have a differential input ADC, you can skip the buffer all together and just use VOUTp and VOUTn directly.  Do not short inputs directly to ground as Alex mentioned.   

Dear Tom and Alex,

Thanks a lot for your information!

But I didn't want to short input directly to GND. Pls see my circuit, I would say it's a pure subtractor, and Vout=VoutP-VoutN, right?

My question is why it's recommended to add the VCM_ADC, and then Vout=VoutP-VoutN+VCM_ADC, what's the advantage for that?

Thanks a lot!

Hi Jin - 

Perhaps what you are missing is what lies beyond Vout in the circuit you provided.  This is what I was trying to explain earlier - if you have an ADC that uses 2.5V as it's common mode input voltage and you only have 1.44 Vcm from the AMC1311, you will loose dynamic range on the ADC.  If you can boost the AMC1311 Vcm through the secondary amplifier to the Vcm of the external ADC, then you can add a little gain and get the full, or nearly full (depends on limitations of a rail to rail amplifier), input range of your ADC - whatever that might be.

Dear Tom,

Now I understand!! Thanks a lot! Thank you for your time and sorry I'm new in this area.

Have a nice evening!

JIN JIAN

No worries Jin,

Happy to help out where we can.  Do let us know if you have further questions.