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I'm using an INA128 as an amplifier with a gain of 100 (Rg = 511 Ω) as in the schematic below (but without the filters.)

I use a signal generator for the input (3 mV.) The expected output is 3 mV × 100 = 0.3 V but I get 1.2 V. I don't understand what the problem is.

Here are the pictures of the schematic, the input from the signal generator, and the output:

circuit schematic

enter image description here

enter image description here

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    \$\begingroup\$ Welcome! Seems your intended schematic didn't show up. Would you re-edit to include it? And something about your test setup would be very helpful. \$\endgroup\$
    – jonathanjo
    Oct 18, 2022 at 8:36
  • \$\begingroup\$ How are the signal generator and scope connected to the circuit? \$\endgroup\$
    – Justme
    Oct 18, 2022 at 8:41
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    \$\begingroup\$ The scope trace is saying ~50 Hz. Is it possible you're picking up mains (and then amplifying it)? Your signal generation says 200 KHz. \$\endgroup\$
    – jonathanjo
    Oct 18, 2022 at 8:42
  • \$\begingroup\$ The in- is connected to ground clip of the signal generator and the in+ is connected to the signal clip! the oscillosope is connected to Vout and ground! Im a beginner so the setup could be wrong im not sure \$\endgroup\$
    – user324158
    Oct 18, 2022 at 8:56
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    \$\begingroup\$ There are no nodes called in-, in+, Vout or ground. So how you connected the devices is unknown. Please, make a description so that another person could replicate your experiment by simply following instructions. \$\endgroup\$
    – Justme
    Oct 18, 2022 at 10:05

1 Answer 1

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Too long for a comment:

First, check the power supply that supplies your +/- 5 volts. The +5 volt and -5v must share the same grounds. This usually means that the black terminal on the +5V must be tied to the red terminal on the -5V. These two terminals are your circuit ground and must also be tied to your signal generator ground. From your scope picture, it appears that your power supply ground is floating. If you are unsure, just add a photo of your setup showing your power supply hookup.

Once you have done this, adjust the function generator down to a lower frequency, say about 2 kHz. The function generator likely has a 50-ohm output impedance, so would need a 50-ohm load to give you the voltage shown on the function generator display. Since you are driving a high impedance, expect the output of the generator to be twice as high as shown on the generator display. So with an output set to 3 mV, you will be applying 6 mV to your amplifier. You should see a 2 kHz signal of 0.6 V p-p.

At 200 kHz and gain of 100, the signal will start to roll off. You can see from the chart below, taken from your data sheet, that the gain is down a few dB at 200 kHz. So the amplitude will start to drop for signals above 100 kHz at this gain setting.

enter image description here

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