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AD8553 Gain always negative

Category: Hardware
Product Number: AD8553

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

    • positive input: VQ5_P = 2.1036

    • negative input: VQ5_N = 2.0805

    • VREF = 0.983V

    • GAIN = 20

    • Expected Output = 1.445V

      • Exp = 0.983 + 20 (2.1036 - 2.0805)

    • Actual Output = 0.45V → even BELOW Vref which indicates something is really wrong

-01 Board Schematic: 

-02 Board Schematic:

Thread Notes

Top Replies

  • Hello,

    I am moving this thread to Instrumentation Amplifiers community. Someone there should be able to assist you.

    Thanks,

    Mae

  • Hi  ,

    Thank you for sharing and posting your inquiries here in the forum.

    To be able to make the proper recommendation, I would like to confirm to you these design requirements and please validate if correct:

    • Single Supply Operation, VCC = 4.25V
    • VEE = 0V or Gnd
    • VINP = 2.1036V
    • VINN = 2.0805V
    • REF = 0.983V, 4.25V thru a res divider ckt (100K||332K)
    • R2 = 100K
    • R1 = 10K
    • Can you please confirm if R66 is open or not being installed?
    • Are both inputs (VINP, VINN) at the DC level only?

    On the DS, the Output voltage is computed using the equation Vref + Gain(VINP -VINN), and based on the details you have mentioned, the actual output voltage you measured was around ~0.45V.

    This suggests that the voltage induced in your REF pin was not accounted for in the output voltage. Please check the Voltage in Pin7 and across R57 (100K) if you can. This should be around ~0.983V.  

    Regards,

  • Hi, thank you for your quick reply! Here is what I can confirm about my design: 

    • Single Supply Operation, VCC = 4.25V
      • Yes. I am using single-supply connection with voltage divider reference. Figure 31 in datasheet. 
    • VEE = 0V or Gnd
      • I don't see VEE listed on the Pinout. Pin 8 (GND) is connected to 0V. Pin 7 (VREF) is connected to voltage divider (0.983V). Pin 3 (VCC) is connected to 4.25V
    • VINP = 2.1036V
      • Yes
    • VINN = 2.0805V
      • Yes
    • REF = 0.983V, 4.25V thru a res divider ckt (100K||332K)
      • Yes
    • R2 = 100K
      • Yes
    • R1 = 10K
      • Yes
    • Can you please confirm if R66 is open or not being installed?
      • R66 is not installed 
    • Are both inputs (VINP, VINN) at the DC level only?
      • Yes. They are DC voltages only. Both vary between 2.05 - 2.2V depending on the sensor signal fed in, but VINP will ALWAYS be bigger than VINN
    • On the DS, the Output voltage is computed using the equation Vref + Gain(VINP -VINN), and based on the details you have mentioned, the actual output voltage you measured was around ~0.45V.

      • Yes, this is correct. My expected voltage was: 0.983 + 20(2.1036 - 2.0805) = 1.445V. But I only measured 0.45V at the output (Vout)
    • This suggests that the voltage induced in your REF pin was not accounted for in the output voltage. Please check the Voltage in Pin7 and across R57 (100K) if you can. This should be around ~0.983V.  

      • I measure pin 7 (VREF) and it measures 0.977V. This is slightly below 0.983V but close enough to be reasonable. 

    QUESTION: What would cause the VREF not being accounted for in the output? Is it possible that the resistor values I have chosen are too high? 

    Thanks! 

  • Hi  

    Thank you for your responses. Let us confirm what other factors in your design circuit might affecting Vref being not accounted in the total output voltage.

    I would request additional validation in the actual circuit:

    On this activity, please isolate AD8553 from other external components (ADC, filters, etc..); please remove a connection to VO5 FILT by removing R77 (143K) res. Remove C19 for the entire measurements.

    • Check the actual Voltage on the Pin4 (VO); if possible, you can capture a scope shot on Pin4 and Pin7.
    • Remove R62, and short Pin7( VREF) to Gnd. Then, check the actual voltage and capture scope shots on Pin7 and Pin4.
    • Remove the Gnd connection to Pin7, Remove R57 (100K res), and connect a separate external supply ~1V. Then, check the actual voltage and capture scope shots on Pin7 and Pin4

    Regards,

  • Please let us know if you can provide these details being requested on your end. For the team to understand your inquiry further and be able recommend the suitable answers.

  • Hi! 
    I changed the Vref from ~1V to Vs/2 (4.25/2 = 2.125V) and that resolved the issue. It seems like at Vref=1V the 1V wasn't getting properly added to the final value, it was only applying the gain of 20. 
    Thanks for your help!