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[SOLVED] Strange behavior of a differential opamp.

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Advanced Member level 1
Jan 22, 2014
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Hello people!

I am using the following current sense opamp from TI: INA293A3 connected on this way


The device is working very nice from when the output of this devide is on the range from 700mV to 3.3V. I measure with the oscilloscope a very nice continues dc voltage waveform. However when the ouput of the device is below than 700mV (it means very low currents), the measurement is a square signal like the one below:


Any idea what can be the reason of such behavior? And the most important. Does anyone know how to mitigate?

Thanks in advance.


First I'd be leaving that one probe on the pulse train,
and start looking for other things in the neighborhood
which might cause it. That would be phase-synchronous
and preceding.

There might be a little bit of switching voltage coupling
that just can't be gotten rid of (close-in, radiated or
conductive). You might, if you find the roughly right
frequency signature somewhere, try things like disabling
its likely source and see if it disappears from the 'scope.

This is old school board level bench debug, have at it.


I checked that the pulses are generated by the opamp. There is not pulses at the inputs and neither the power supply ...

It looks like the opamp is working like an aestable for very low currents...but i don get it because this opamp is designed specifically for this applications....



Tbh, I think the pulses are true because of two reasons:
* It's quite common for a LED supply to pulse at about 200Hz. Mind: "current pulses", this does not necessarily mean that the voltage also pulses 0% to 100%
* It´s quite unusual for this "currentsense amplifier" (it´s not an OPAMP) to oscillate on their own. And then for sure not at a that low frequency.

I´m rather surprised that it sends out nice, flat DC (unless the brightness is at 100%)

Especially on a LED driver I always would use an averaging filter (... like shown in Fig 8-1 of the datasheet.)

For further discussion we need your complete schematic, including LED driver and LEDs.


Hi! I have already solved the problem. It was correct the pulses were at the input of the current opamp and they are magnified because its 100 gain. Changing the working mode of the LED driver solved the problem...
thanks for your help
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