AD7764 Need help understanding the internal differential amplifer and datasheet.

Hi RedshiftBioEE ,

The input +/-2.5V centered around 0 is the input applied before the external resistance R_IN (points A and B), while the common mode input of 0.8 to 2.2V, is referring to the inputs of the differential amplifier, VINA+ and VIN-. 

If your input to both VinA+ and VinA- is 0V, both VoutA+ and VoutA- will be 2.048V.

Thanks,
Janine

Thank you for the response. This makes sense to me, I wasn't thinking about the voltage drop over RIN. 

This means that the circuit that I designed to interface with the ADC should be correct. My problem still persists. The problem I am seeing is that if there is 0V at the input (positive and negative side) (before RIN) then at VINA+ I am seeing ~4V at VOUTA+ and 0.5V at VOUTA- giving me a differential voltage that is large and out of range of the ADC. I think I should see 2.048 volts at both nodes, and a differential input of 0V. Can you think of anything that would be causing this? Schematic is below. 

/resized-image/__size/640x480/__key/communityserver-discussions-components-files/425/pastedimage1681823632254v1.png

Hello,

when I download the png of the schematic, I can hardly recognize the pin-numbers the net-names. Don't know, if this is a display artefact on my side or if it's due to some rescaling of the uploaded picture.

But from your failure description I would recommend that you doublecheck, that the external components of the diffamp really give you a negative feedback. So: what is the value of R_in, what is the value of R_FB, does one R_FB really go from Pin 2 to Pin 1 and the other from Pin 4 to Pin 3 (not mixed up)?

best regards

Achim

Hi Achim,

Thank you for the response! Even without being able read the schematic you nailed it in finding my issue. For completeness I tried again to attach the schematic, but yes my issue was that I had the feedback resistors for the differential amplifier connect VINA-(pin 1) to VOUTA- (Pin4) and VINA+ to VOUTA+ ( pin3 to pin 2). Once I swapped them things worked correctly. Thank you!

/resized-image/__size/2598x1346/__key/communityserver-discussions-components-files/425/7271.ADC.png

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Hello,

RedshiftBioEE said:

For completeness I tried again to attach the schematic

Thanks, this time I can read the schematic clearly.

RedshiftBioEE said:

I had the feedback resistors for the differential amplifier connect VINA-(pin 1) to VOUTA- (Pin4) and VINA+ to VOUTA+

so you had positive feedback instead of negative feedback. In that case, the smallest voltage difference at the input will be increased by the feedback until the amplifier output reaches its limits.

RedshiftBioEE said:

Thank you!

you're welcome...

Achim

Hello,

RedshiftBioEE said:

For completeness I tried again to attach the schematic

Thanks, this time I can read the schematic clearly.

RedshiftBioEE said:

I had the feedback resistors for the differential amplifier connect VINA-(pin 1) to VOUTA- (Pin4) and VINA+ to VOUTA+

so you had positive feedback instead of negative feedback. In that case, the smallest voltage difference at the input will be increased by the feedback until the amplifier output reaches its limits.

RedshiftBioEE said:

Thank you!

you're welcome...

Achim