INA293 shunt amplifier response delay/confusion

[uer166]

I have a boost converter with a 5mOhm shunt amplified by a INA293B2 amp. The first single pulse tests went good regarding power stage but I see a 5us 10us delay from when the current starts ramp, to the INA293 very quickly ramping up to the correct voltage. So far I've checked:

• Input offset: 80uV worst case, which cannot explain such a large delay/offset. In my trace the output corrects itself at ~0.25V out, which would correspond to 0.25/50=5mV offset??
• Settling time: doesn't apply since this is a nice linear ramp from 0, but still it's ~1us, not 10
• Power rail is perfectly flat during the pulse at the amp

I had the same problem with the internal STM32F3 opamps, surely thought this amp would fix and actually track current like I require. What did I miss?

[StillTrying]

It does look like an op amp coming out of saturation to me. Is this something to do with it, page 15 on the data sheet.

"The linear range of the output stage is limited to how close the output voltage can approach ground under
zero-input conditions. The zero current output voltage of the INA293 is very small, with a maximum of GND + 20
mV. Make sure to apply a differential input voltage of (20 mV / Gain) or greater to keep the INA293 output in the
linear region of operation."

I don't know the solution if it is.

[uer166]

Ah, I think this is related to the issue, which is vaguely similar to the output linear operation but it's to do with the input:
"8.2.2.1 Overload Recovery With Negative VSENSE
The INA293-Q1 is a unidirectional current sense amplifier that is meant to operate with a positive differential
input voltage (VSENSE). If negative VSENSE is applied, the device is placed in an overload condition and requires
time to recover once VSENSE returns positive. The required overload recovery time increases with more negative
VSENSE."

Really wish TI gave actual numbers here.

I think any input voltage of 0V + Vos (the 80uV or so) can be considered negative, depending on that opamp's input offset. I've biased the +ve input to 180 uV and now the issue is gone. I think in next rev I'll bias it to the 20mV/gain to be 100% sure, this will require extra series resistors between shunt and INA inputs..

[StillTrying]

It's amazing how hard you have to study a data sheet, all the while thinking 'what if'.