Problem with reference voltage

Discussion in 'General Electronics Chat' started by msr, Sep 22, 2010.

  1. msr

    Thread Starter Well-Known Member

    Jul 8, 2008
    Hi guys,

    I want to amplify a signal and "center" it in 1.65V (3.3/2). For voltage reference Im using a simple voltage divider with two 100k resistors and a buffer (with a LM324 opamp). Im using a INA122 (instrumentation amplifier) to amplify and get a biased signal in 1.65V.

    For small gains (~10) it works. But I want greater gains (>100) and in that situation Vref starts decreasing (until GND). Why this happens? What can I do to avoid this?

    I also tried to use a common inverting amplifier circuit with a LM324 opamp but the Vref also decreases when gain increase.

  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Is the signal DC coupled to the amp?

    A schematic drawing of your circuit would be a big help for answering questions on this problem.

    Try letting more current flow in your ref divider. Go with 10k instead of 100k.
  3. timrobbins

    Senior Member

    Aug 29, 2009
    I agree about the 100k to at least 10k - as that node is being pulled by the 100k in the IA.

    Also, you're not railing the output are you?
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    Why not just pick up a voltage reference IC in that value, or at least adjustable to it?
  5. msr

    Thread Starter Well-Known Member

    Jul 8, 2008

    The problem is not with voltage reference. Im also amplifying offsets and before INA122 I have a high pass filter (2nd order butterworth) removing DC component which maybe isn't removing it all. The signal I want to amplify has ~20mVp-p. So thats the problem, I think.

    How can I minimize the effects of offsets? How to "center" the signal on 1.65V (for any gain)? My purpose is to use (aprox.) rail-to-rail output and then get a better resolution after ADC.

    Maybe changing Vref of INA122 (with help of a MCU) in acordance to selected gain?

    Thanks for your help!
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    I would say that a schematic and details are in order.
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You said you have a highpass filter in the signal path. If your ADC has a high impedance input, just AC couple between the IA and the ADC. This will be your highpass filter.
    The advantage of using a voltage divider (instead of a voltage reference) is that your half-scale biasing will track changes in the ADC reference voltage. Use 1% resistors for stability. See attached circuit.
    If your ADC does not have high input impedance, this won't work.
  8. msr

    Thread Starter Well-Known Member

    Jul 8, 2008
    RonH, I can't use the HP filter after IA because de signal I want to amplify has a large DC component. It has ~1V DC and I want to amplify the AC component that is about 20mVp-p.
    But thats a nice idea! I can use that HP filter with bias to compensate the offset that is amplified, and thus repositioning the signal in 3.3V / 2. I'll give it a try!

    Later i'll upload the schematic but thanks for the replies until now!
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Why do you need an active HP filter? Can't you AC couple it twice - once in front of the IA, and again in front of the ADC (with the 3.3V/2 bias)?