I have been using the AD8428 to amplify fast transient signals in the 10-100 uV range with microsecond time resolution. The AD8428 so far appears ideal for this purpose with settling time less than 1 us at a gain of 2000, but I seem to have destroyed one chip as the output now always reads +6V even with inputs shorted to ground (+/- 9V supply). I have read other questions here where the fragility of the chip has been brought up so I want to take some precautions to avoid breaking more as they are somewhat expensive.
Can the AD8428 be damaged when inputs are left unconnected (i.e. floating) intermittently? My application is essentially a four-wire low value resistance measurement with a portable differential voltage probe connected to the inamp, thus the inputs are not permanently connected to the sensor. Normally, when the probe is connected to the specimen, the source is not floating in the sense that there is a bias current return path to ground through the probe in contact with specimen.
What is the recommended method to deal with intermittent floating inputs (e.g. to allow for sensors to be connected/disconnected while amp is powered on)? With other amps I have connected each input to ground via large value resistors, but while experimenting with the AD8428 I had to reduce the value of these resistors down to ~100 Ohms to avoid oscillations at the output with probe disconnected. However the low value resistors, while still large compared to the source impedance (~100uOhm) seemed to cause variable common mode errors depending on the contact between the probe (spring-loaded pins) and the specimen. Incidentally it appears I broke the amp after removing these resistors which is why I suspect that floating inputs might damage the device.
By the way I did consider the AD8421 as a more robust alternative, but at the moment I can not find it in stock anywhere.
The parts usually get damage if you violated any of the absolute max rating shown below. Floating inputs should not damage the part though this can make the AD8228 output to go into the supply rails and overdrive the next stage after the AD8228.
Typical solution, as you mentioned is putting bias resistor to ground. What do you mean reducing the value of the resistors to avoid oscillations? Are you referring to noise at the output due to the large value of resistor? What is your operating frequency and can you implement filter at the output to lessen the effect of noise?
Do you also have any other components at the inputs aside from your sensors/probe?
Thank you for your reply.
As you can tell from my questions I am an amateur in electronics and you will not be surprised to hear that my problem was caused by a real rookie mistake: when I desoldered the resistors to ground I had shorted one input to one of the FIL terminals. The part works as expected again after fixing this.
What I meant by oscillations in my original questions was that the output oscillated rapidly between +/- Vs for ~200k resistors to ground. Had to reduce values to ~100 Ohms to get a stable output without sensor connected (there are no other parts other than probes connected to input). However I observed this while prototyping on a breadboard and I guess stray capacitance could also cause problems in this case due to the high gain and bandwidth.
After taking your reply into consideration I decided that leaving the inputs floating occasionally will be fine for my project and I simply leave out the resistors to ground which eliminates the variability in my measurements. Will take care not to exceed the max ratings on the input, in particular when the chip is not powered on!