I used the AD8553 to produce a Gain = 20 for a -01 version of my board. For the -01 version I originally had VREF=0V so I had to rework the voltage divider to produce VREF=0.983V to be above the 0.8V minimum listed in the datasheet.
For the -02 version of the board, the circuit is IDENTICAL except:
1) I implemented the above rework (VREF = 0.983V) to be a permanent part of the design
2) I Loaded the bypass cap C19. I have tried removing this cap and results were still the same.
My results for -01 board were great. But for the -02 board the gain is negative when the positive input is greater than the negative input. AND the gain is also inconsistent, as you change the negative input the gain magnitude changes.
My questions are:
1) Is there anything in my new schematic that you think is causing the -02 version to fail when the -01 version worked?
2) What would cause the Op Amp gain to be negative even if the positive input (VQ5_P) is greater than the negative input (VQ5_N)?
Thanks!
-02 RESULTS: FAIL
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positive input: VQ5_P = 2.1036
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negative input: VQ5_N = 2.0805
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VREF = 0.983V
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GAIN = 20
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Expected Output = 1.445V
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Exp = 0.983 + 20 (2.1036 - 2.0805)
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Actual Output = 0.45V → even BELOW Vref which indicates something is really wrong
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-01 Board Schematic:
-02 Board Schematic:
