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INA122: INA122U

Part Number: INA122

hi,

The circuit adopts positive and negative 9V power supply. When the power supply has no obvious glitch, the chip works normally.

When the power supply has large glitches (peak-to-peak value is 720mV, glitch cycle is 16.6us), the voltage on pin 6 of the chip is not stable.

Use a multimeter to measure:
Pin 6 has an unstable voltage, sometimes the output reaches 2V, sometimes the output reaches -2V.
When moving the meter, there are fluctuations of several tens of mV. Even if you do not move the meter, the voltage will fluctuate and become unstable. In theory, 0V should be output. The multimeter is working properly.

Observe using the oscilloscope, pin 6 output voltage is still 0V, but there is a large glitch at 0V, glitch voltage peak-to-peak about 620mV.

problem:
1 In this case, why the output voltage of Pin 6 is unstable (measured by the multimeter), and the waveform observed by the oscilloscope is normal.
2 In addition to reducing the glitch voltage, are there other solutions to this problem? Make INA122U output voltage normal, no abnormal fluctuations.
3 This phenomenon is not determined by the internal circuit characteristics of the INA122U.
4 If  do not use the INA122U, please recommend an instrumentation amplifier with other signals. Magnification G=200K/Rg 1

  • Hi Eric,

    first, the supply voltage of INA122 should not show any large glitches. You need a well regulated supply voltage without any glitches. And you should have mounted power supply decoupling caps in your design like shown in figure 1 of datasheet.

    That the glitches appear at output of INA122 can have to do with the finite power supply rejection ratio (PSRR) of chip. Each chip can suppress noise on the supply lines superimposed to the supply voltage only to a certain degree. That's the reason why the supply voltage should be clean and free of glitches.

    That you get different measurement results with the scope and with the multimeter can have to do with the averaging effect of multimeter.

    Eric, you know what to do. You told it by yourself when mentioning: "When the power supply has no obvious glitch, the chip works normally." :-)

    Kai
  • Eric

    We haven't heard back from you so we assume this answered your question. If not post another reply below, or create a new thread if this one has timed-out.

    Thanks
    Dennis