In-Amp Variable Reference Voltage as Active Buffer

Discussion in 'The Projects Forum' started by witssq, Mar 5, 2011.

  1. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    I would like to make a circuit of low noise adjustable reference voltage to instrumentation amp.

    Using the attached doc's figure 11 "An Op Amp Buffer Connected as an Active Filter Drives the Reference Pin of an In-Amp", Not destroying purpose, I would like to
    adjust reference voltage from -Vs to +Vs(dual supply rail).

    The simple expected revision is

    1) C2(1uF) to GND -> C2(1UF) to -VS (for dual supply)
    2) R2(200kOhm) to GND -> R2(200kOhm) to -VS (for dual supply)
    3) Supply rail GND -> -VS
    4) R1(200KOhm) and R2(200KOhm) -> (20KOhm) + Trimpot(400KOhm) + (20KOhm) (Fixed to Adjustable, 90% of supply rail)


    Could anyone help me about other component calculation and the above approach.


    Best regards,

    SunSung
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    Those changes make sense to me. What are you having trouble with?
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The whole point of the active filter is to remove power supply noise from the reference created by the voltage divider. When you return C2 to -VS, you defeat this purpose. Noise on -VS will appear at the op amp output unattenuated. Use a nonpolarized 1uF cap, connected to GND as shown.
     
  4. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Many thanks for your advices.
    After testing this circuit, I will respond with result.

    I have one more question about REF Voltage.
    I would like to know about exact transfer function of AD8221 with REF voltage term included.
    I expect that it is : VOUT = ((VIN+ - VIN-)/(1 + (49.4K/RGAIN))) + VREF(REF Pin Voltage).
    But on test, It seems not to be correct especially on REF term.

    Please help me about this issue.

    Best regards.
    SunSung
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I don't know any more than what the datasheet says, but it is probably correct. Did you measure Vref at pin 6 (REF)?
     
  6. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Thanks,

    Yes, VREF(TP2) on the attached schematic is correct with +/- 8V swing.
    It seems that Transfer function is not simple addition of VREF.

    Although doing more test for quantifying, if you have some idea about
    this. please comment.

    Best regards,
    SunSung
     
  7. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    It seems that the above transfer function is correct.

    Problem is different point
    VREF swing should be above +/- 15V to amplyfy input (12.4-71.1mV) to output(-10V - +10V) with -42.7mV as Zero Point, as shown on the attached excel file image.

    But REF pin cannot accept above Vs(+/- 10V).

    Without enlarging supply voltage swing, How could I solve this problem?

    Thanking you in advance.

    Best Regards,
    SunSung
     
    • AIR.jpg
      AIR.jpg
      File size:
      296.2 KB
      Views:
      27
    Last edited: Mar 7, 2011
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I don't understand your problem description or your spreadsheet. Can you post the equation for the transfer function that you need, and the schematic of what you have?
     
  9. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Sorry, I missed then schematic upload.

    My transfer function is the above reply. It is correct.

    I need to make VDIFF(-42.7mV) to VOUT(Zero) on excel sheet.
    To establish this level shift, +15V level shift is needed.
    But level shift on reference pin more than +10V is not allowed referring to
    data sheet.

    Best regards,
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    But that is not an equation. What I meant was something like this:
    Vout=kVin+Voffset
    Where you fill in k and Voffset.

    From your spreadsheet and what you have said,
    Vout=341.690*Vin+14.590

    Is this correct? If so, you need to run the IA at lower gain and offset, and then post-amplify the output with a conventional noninverting op amp stage.
    For example:
    Vout=10*(34.169*Vin+1.459)
    In this example, the post-amp would have a gain of 10. You can make the post-amp gain any value that will allow you to keep the IA offset below 10V.
     
  11. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Many thanks for your help.

    Transfer function you requested is
    VOUT = ((VIN+ - VIN-)/(1 + (49.4K/RGAIN))) + VREF(REF Pin Voltage).

    You seems to understand correctly spreadsheet.

    I would test your suggestion about post-amp.

    About +/- 5V output swing can be achieved current schematics. post-op amp for +/-10V seems to make more noise between AD8221 and ADC thant direct conversion with +/-5V. Is this correct?

    Is thera a possibility to solve this problem using some network between AD8221 output pin and REF pin?

    Best regards,
    SunSung
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Do you have an A/D that can accept ±10V input?
    How many bits is it?
     
  13. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Yes, it is ADI
    AD7734 24 bit.

    regards,
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Sorry for all the questions, but I'm trying to get a handle on the noise question you asked.
    What is your system bandwidth?
     
  15. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Thanks,

    Please let me know about what system bandwidth means, because I don't have much experience.

    Best regards,
    SunSung Hwang
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    What is generating the voltage you are amplifying and digitizing?
     
  17. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    It's pressure sensor with bridge.
    Please refer to the attached docs.

    Best regards,
    SunSung
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    There are no documents attached.:confused:
    What is your A/D sample rate?
     
  19. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Sorry,
    I'm attaching documnets
    I'll return with sample rate.

    Best regards,
    SunSung
     
  20. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    The sample rate to need is maximum of AD7734

    It is about 6MHz(AD Master CLock) x 200 = 1.2KHz

    I open new thread for noise reduction of this circuit.
    If you also help me, please refer to it.

    Best regards,
    SunSung Hwang
     
Loading...