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I built a circuit based on the INA128 which for some reason heats up rapidly. The test setup is like the image below. The output is left unconnected, no input signal. I get around 550mV voltage drop on R6. With the Vref pin unconnected the voltage drop on R6 is 45-55mV so the IC doesn't heat as much.

circuit_1_test

At first I though I messed up with the Vref source so I modified the circuit like this but the result was the same. I tried this with two ICs and got the same result.

What am I doing wrong?

circuit_2_test

Edit 1: Both images replaced with hopefully more readable schematics.

Edit 2: Some more info. I intend to use this circuit in a little car audio experiment hence the requirement for single supply. The input signal is differential audio that is coupled through two capacitors to R7 (R15 second image) and R8 (R16 second image.) The purpose is to convert from differential input to single ended output with some gain (g = 10 in this case.) The coupling capacitors are not present both on the schematic and on the prototype board that I built. That is why it seems the inamp's input is just noise as @kruemi suggests. With just this minimal setup (no input signal, output unconnected, Vref present) somehow the inamp is drawing excessive current. I will try to check with an o-scope for any oscillation and post the results.

Edit 3: The output of the TL431 in all tests (with and without TL074 opamp / with and without INA128) is 6.104V. I did not observe any oscillation at Vref or Vo pin of INA128. Vo seems to be drifting between 6.6-6.8V regardless of any input signal except when I press the top of the IC with my finger to check temperature - then it goes to 5.5V. I tried increasing R6 (R14) from 10 to 100 ohm - current consumption reduced somewhat to 42mA. I also tried the prototype with a dual power supply (5V and 12V, Vref connected to midpoint.) The only difference is at +-12V the IC heats up faster.

Edit 4: I have ordered some new INA128 from a different vendor. Will update once they arrive.

Edit 5: After a long, long shipping time I finally received some new INAs. Everything works fine with these new ICs, no overheating issues. Its seems the ones I had before were all defective.

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    \$\begingroup\$ Reading your shematics is really hard... please try to draw them left-to-right and top-to-bottom in the future... So, have you measured anything? What are the voltages? A wild guess: something is oscillating, you can check this with an oscillsocope. \$\endgroup\$
    – kruemi
    Mar 8, 2022 at 14:09
  • \$\begingroup\$ Are you required to use single (unsymmetric) supply only? \$\endgroup\$
    – LvW
    Mar 8, 2022 at 14:12
  • \$\begingroup\$ So right now you seem to by trying to amplify the noise of the 1M-Resistor at the input by a factor of 10. Ist this right? What is the voltage you get when you measure DC at "ref". And what do you get if you set your multimeter to AC there? Do you have an oscilloscope? \$\endgroup\$
    – kruemi
    Mar 8, 2022 at 14:48
  • \$\begingroup\$ What is it supposed to do? \$\endgroup\$
    – Polynomial
    Mar 8, 2022 at 17:56

1 Answer 1

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If connecting the "REF" pin causes an amount of excess current in the order of 50 mA, that sounds like you accidentally confused it with the output pin.

Secondly, does the TL431 output measure correctly (~6V) when not connected to the INA? I ask because there are pin-mirrored version of the TL431 out there, so possibly another cause for problems. If the TL431 output is alright then I suspect a damage of the INA 128. Possibly, the internal ESD diode from the REF pin to ground could be shorted.

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  • \$\begingroup\$ I checked but no this is the INA128 pinout from the TI datasheet. This is my layout with the Vo net highlighted. The silkscreen names do not correspond to the given schematic. R5 is used as a jumper and is currently unsoldered so the output is left unconnected. \$\endgroup\$
    – widis
    Mar 9, 2022 at 7:55
  • \$\begingroup\$ @widis Yup, it looks alright. Does the TL431 output measure correctly (~6V) when not connected to the INA? I ask because there are pin-mirrored version of the TL431 out there, so possibly another cause for problems. If the TL431 output is alright then I suspect a damage of the INA 128. Possibly, the internal ESD diode from the REF pin to ground could be shorted. \$\endgroup\$
    – tobalt
    Mar 9, 2022 at 8:17
  • \$\begingroup\$ How can I check for this ESD damage? \$\endgroup\$
    – widis
    Mar 9, 2022 at 8:54
  • \$\begingroup\$ @widis when not powered and with nothing connected to the REF pin, measure resistance between the REF pin and negative supply pin. \$\endgroup\$
    – tobalt
    Mar 9, 2022 at 9:58
  • \$\begingroup\$ Vreft is stable at 6.104V, no oscillations. There is no short between Vref and negative supply. I checked both directions with a multimeter and got megaohms of resistance. Same thing between positive and negative supply. TBH I'm starting to think I got 5/5 defective IC. \$\endgroup\$
    – widis
    Mar 9, 2022 at 11:56

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