Problem with reference voltage

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

  1. msr

    Thread Starter Active 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

    Active 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 Active 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 Active 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)?