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OPA1632: Output balance problem (single to dif conversion for AD inputs)

Madis Reivik
Prodigy 80 points
Part Number: OPA1632
Other Parts Discussed in Thread: OPA1655

Tool/software:

Hello !

I get different p-p amplitude from the outputs.(check the red remarks 1860 vs 2120mV)

I have checked following:

1) checked and doublechecked all resistors - nope

2) tried to add unity buffer to filter circuit output - nope

3) changed gnd-signal inputs in OPA1632 - outputs also switched places as they supposed to do :) along with the unbalance

4) tried Vcom input - open, GND and Vref - nope, no effect to output gain balance

5) removed the 1632 capacitors - nope

5) disconnected the AD converter from OPA1632 - no effect

Is there some connection to previous stage opamp output impedance ? The error is too big IMHO for that. Im going crazy, tried almost everything. At least, good to know that I have not mistaken in prototype assembly.

Please share experience !

Also, is the imbalance problem at all (exept that AD input range is a bit reduced) ?

  • 0
    TI__Mastermind 21402 points

    Hello Madis,

    I will help discover what is keeping this circuit from operating as intended.  

    Would you share what the gain-setting resistors would be for the OPA1632?  I see the inputs being driven by OPA1655; are both inputs driven by the OPA1655?  Are there resistors helping to set the attenuation on the FDA?

    Can you share your desired input and output voltage so I can simulate with OPA1632?

    Best,

    Alec 

  • 0 in reply to Alec Saebeler
    Prodigy 80 points

    Sorry, there was one resistor missing between 1632 and 1655 (it is present in original schematic).

    I made 2 corrections with MS Paint

    The other input is  grounded with 1k resistor.

    gain resistors are 1k and 499R. Input voltage is about 6V p-p.

    I suspect that the output resistance of 1655 is playing some role... but there should be none ?!?

    Any ideas ? :)

  • 0 in reply to Madis Reivik
    TI__Mastermind 21402 points

    Hello Madis,

    Yes, I am wondering the same about 1655 driving into 1632.  Thank you for clarifying the gain resistor settings.

    Let me take a deeper look into OPA1632 performance and if the impedance on the inputs can cause this large of a performance shift.

    If we increased or decreased Rf & Rg to maintain the same gain, I wonder if the resistors (originally 1kOhms) loading the OPA1655 would contribute to the magnitude of the offset.  For example, right now Rg = 1kOhm and Rf = 499 Ohm.  If these were doubled to Rg =  2kOhm and Rf = 1kOhm, would this change the apparent offset between OUT+ and OUT- on the OPA1632?

    Best,

    Alec