Hello Dear Community,
We are using AD629 (will be replaced with AD8479 in our next revision for its rail-to-rail output) to measure high voltage (5000V - DC & PWM -). Bellow is the schematics of our implementation:
During the first test, we wanted to validate the AD629 implementation. So, we injected a small signal (0-2.5V) that has 1us rise/fall time, and 30us on/off time.
As result, we have seen oscillations on the output (around 250KHz) as you can see in the below scope traces. We got the same results even when disconnecting the following stages (AD8138...).
Same results is observed when using small signals at the input (50mV):
We decided to build the same circuits on a test board. The results was the same.
What do you thing about this problem? Is it because AD629 has kind of high slew rate or maybe because of its slow settling time... maybe both or some other factors can cause such an issue?
Please, help it is kind of urgent.
What does the bottom of this board look like? I am particularly interested in how the bypass caps on the positive and negative supplies tie back together. If you have a leaded 0.1uF cap you could try soldering that right across the top of the part between the supplies. Also, is you reference supply also decoupled at the board (its not clear how pin 5 is connected to the 5V reference on your board)?
The bottom side is a full GND cooper plan. each connection to GND (bypass cap and others) is made with short lead-wire to this bottom GND-plan. Sorry I forget to update the schematic: the current version both ref are tied to GND and the AD629 is dual supplied +/-10V. But, thank you to notice that in each configuration (dual supply 0V-ref, single supply 5V-ref) we have got the same results
Now LTSpice schematics is updated