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Using INA122 for nulling other amplifiers.

Smiffy
Prodigy 90 points

Other Parts Discussed in Thread: INA122

I want to put together an instrument that I can use for setting the offset null of op amp buffers in other circuits, as well as checking small voltage differences in other circuits, where the measuring equipment would need to have a very high input impedance.

My idea was to use an INA122 (which I have to hand) to amplify the difference between the input and output of the amplifier on test. This would drive a centre-zero galvanometer, so I would adjust the nulling pot on the amplifier under test until the meter read zero.

As I was going to power this from a single 9V battery, and because the difference being read could be either positive or negative, should I connect the REF pin of the INA122 to a buffered VCC/2?

Secondly, to provide a bias current return path, should I connect resistors from both inputs to the V- pin and, if so, what value should I use?

  • 0
    TI__Mastermind 25865 points

    Hello Smiffy,

    One concern is the input common-mode vs. output voltage range.  Note that you will lose some output voltage swing when you connect Vref to Vcc/2.  Please see the lower-right figure on page 4 of the data sheet.  Nonetheless, you will have to connect a reference voltage in order to measure both positive and negative voltages.

    Concerning the input bias current return path...the resistors are required when interfacing with very high-impedance sensors/circuits as shown in Figure 3.  Provided there is any path for current to flow you most likely do not need the resistors.  

    Hope this helps.

  • 0 in reply to Pete%20Semig
    Prodigy 90 points

    Thanks, Pete. 

    In this case, a loss of output swing isn't a big issue, as this instrument is all about detecting a null point - the most important output voltage being zero (circuit under test is set correctly.)

    Just out of curiosity, if I output swing were critical, should this be done with a dual supply?

    Cheers

  • 0 in reply to Smiffy
    TI__Mastermind 25865 points

    Hello Smiffy,

    If the application requires the output swing to include or go below ground, a dual supply is necessary. Put another way, with a single supply the output of the INA122 can only come to within 150mV of ground before becoming non-linear.

    I hope this helps.

  • 0 in reply to Pete%20Semig
    Prodigy 90 points

    Thanks, Pete. I'll go the dual supply route - it's only a case of adding a second battery snap.