I use the AD8422 In-amp in two different places in my board. Both AD8422 uses dual supply of about -12V and 12V. Both are connected to an analog switch and then to the input
of a LT1468 that is used as a buffer to drive an ADC.
One is used to measure a voltage within -5V to 5V and the other one to measure a current -50mA to 50mA by sensing the voltage over a 100R resistor.
Both get oscillations at around 750kHz at some voltage/current levels. For example the voltage output is good between 0-5V but oscillations occur around -2V, but not for the complete range -5V to 0V.
The current measurement was oscillating all the time if I remember correctly.
At the output I used a 50R resistor and then 100pF to GND.
Increasing the 50R to 10k oscillations stopped, using 1k still no oscillations. With 500R it start oscillate again.
Finally I used 1k and 1nF to get a suitable low-pass filter for my application and in this case no oscillations either.
The question is why I see these oscillations from time to time and most importantly how I can make sure I not get them.
For the current measurement case I also tried to change the gain from 1 to 2 before changing the low-pass filter on the output, but no change in oscillation freq or amplitude.
I got the same behavior on two different board with the same solution (increase resistance in output low-pass filter).