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INA181: Overheat

Adriano Petrucci
Prodigy 20 points
Part Number: INA181


Hi,

I am trying to implement this amplifier to detect a low current 1/100 V in a circuit.

I want to detect a variable DC (0 to 5V) from 1mA to 200mA and also direction on a line with up to 1A (everything over 200mA is not relevant).

So I power it with 5V and the reference is VCC/2 (2x10k resistors). The shunt resistor is 0.1.

Now, since the circuit can be disconnected, the In+ is "floating" (just connected to the shunt) and In- has just a pull-up of 50k. So, no current is detected.

It works, but can't understand why it become so hot just after a few seconds. I was thinking that it would be able to detect also a 0mA current.

Maybe I selected the wrong amplifier, but was sure it should work.

Thanks, Adriano

Edit: added circuit (ATMEGA88 has a pullup of 50k):

  • 0
    TI__Mastermind 32395 points

    Hey Adriano,

    Welcome to the E2E forum.

    I'm not sure what exactly could be happening, but I will try to give a couple sequential debugging steps.

    1. Make sure the bus ground is connected to the digital ground.

    2. Replace C10 with a 1nF capacitor. This 100nF cap is most definitely forcing the INA181 into instability.

    3. Try repeating the test with PC3 completely disconnected from circuit and/or set this pin to ADC input and not as the 50k pull-up.

    I would also note that the INA181 input bias currents (IB) into the device input pins will be between 15uA and 60uA. These current will be sourced from PCINT11 when pull-up is activated. This means that there will be a voltage drop across the pull-up of IB*50k. I don't know how this could heat up the part, but I figure I would mention this to you.

    Also, when circuit is disconnected, both input pins (IN+ and IN-) will be pulled up through PCINT11 since they are connected with small shunt resistance.

    Sincerely,

    Peter Iliya

  • 0 in reply to Peter Iliya
    Prodigy 20 points

    Peter Iliya said:

    2. Replace C10 with a 1nF capacitor. This 100nF cap is most definitely forcing the INA181 into instability.

    Thank you very much!

    It works like expected now, thank you!

    I removed the C10 (since the current is more or less stable, I was thinking this was a good solution, instead of a low pass filter, but wasn't :P) and the ina181 didn't heat up anymore.

    Before, I had ~15mA of current drain over this IC. Now, even without filter, I have a fantastic signal :D Thank you!

    Note:

    1- I forgot also to connect AVCC to VCC, but this was not relevant for my problem.