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Amplifier for chemical oscillator

Sam Kenny
Prodigy 60 points
Other Parts Discussed in Thread: INA116

Hi, I'm a chemist at the University of Minnesota studying oscillating chemical reactions.  I'm using a Pt electrode to electrically monitor the oscillations, and a Vernier voltage probe to record the oscillator voltage over time.  The chemical cell produces roughly a sine wave with a period of 100s and an amplitude of 50mV.  I'd like to pre-amp this signal to ~5 or 10V to make up for the low precision (~10mV swings) of my voltage probe.  I'm thinking an instrumentation amplifier is the way to go; any thoughts?  I'm worried about how the amplifier will handle such slow oscillations.

  • 0
    Guru 21975 points

    Sam,

    An instrumentation amplifiers seems like a sensible approach. I'm not sure how the input signal is referenced. Are you measuring with respect to ground or to some second electrode that is not at ground potential? An instrumentation amp would be best for the latter situation. If the input electrode is always measured with respect to ground, a single op amp may be adequate.

    A key consideration is the input bias current of the amplifier you select. The input bias current is the current that will flow in your electrode. Depending on the nature of your cell, this current can polarize the electrode and disturb the reaction. We have op amps and instrumentation amplifiers that span a very wide range of input bias current, from nanoamps to femptoamps.

    So far as the frequency of oscillation, I see no problem. As long as you do not use AC (capacitor) coupling in the amplifier chain, it will have response down to DC.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 60 points

    Thanks, Bruce - the bit about input bias current is good to know.  The cell does seem to be quite sensitive to applied currents. 

    As far as referencing, I'm currently referencing the cell against a standard Zn/ZnSO4 cell, which runs at about 1.5V.  I'm using a AA battery and a 100kohm pot to zero out the baseline voltage.  Any concerns with zeroing out the voltage before connecting to the instrumentation amplifier?  Otherwise, I'll go ahead and look for a low input bias current amplifier.

     

    Thanks!

     

    ~Sam Kenny

  • 0 in reply to Sam Kenny
    Guru 21975 points

    Sam,

    You may get less noise pickup by connecting the common side of your cell to ground. Then reference the baseline offset circuit to the same ground with its output to the opposite input of an instrumentation amplifier. This way, both are ground referenced and there is no floating midpoint voltage. The offset circuit should have a capacitor to ground to filter the noise from the potentiometer.

    INA116 is our instrumentation amp with lowest input bias current.

    Regards, Bruce.