I am trying to use AD8307/AD8310 (I tried both on different boards) to detect the shape of 50-400us pulses (DC pulse). I am using the chips in DC connected configuration, signal placed at the bias voltage advertised in the datasheet (3.1V for 5V supply), large 10uF capacitor on offset pin and connected to a 12 bit 2MSPS ADC, buffer at input and output before ADC. Everything is working fine as expected except random ripple/oscillation on both chips AD8307 and AD8310.
The oscilations have an unusual/unexpected shape and resemble a RC discharge curve with rapid buildup and in my case discharge with tau around 1 us in my case. By unusual I mean, that if the whole chain of the amplifier cells in the chip goes to oscillation then I would expect either something like "log sin(t)" shape or at least sin shape if none of the cells goes into saturation/limiting. Which leads me to the conclusion that the chain of amplifiers is not oscilating, but rather something else in the chip and there are two possibilities - either the output interface or the offset, where both have at least some capacitance, which could cause this behaviour. By the way, the feature is a bit chaotic, frequency depends on level of signal (low signal clearly shows more oscillation, but I am getting also bursts of oscilations at input level in 10's of mV - burst = they appear and go away in matter of say 10-15 us, whereas at low level signal I can have sustained oscillations, the frequency is typically somewhere between 300-500 kHz, but changes).
I was reading through this site and it seems that I am not the only one observing this behaviour and I am linking just a couple for reference:
For me the problem is that the oscillations destroy the response at low levels and basically make the design/use of these parts not feasible. So my main question is - do you know what might be the likely source of the problem? I.e. source/reason for the RC ripple.
I beg your pardon for the delayed response.
AD8307 and AD8310 are very high gain, very sensitive devices as you know. They will respond to outside noise from things like switching power supplies nearby. To help troubleshoot this type of problem, we recommend to use all linear power supplies for the purpose of this testing. Make sure no ground loops are possible on the input.
You might try disconnecting the detector output from the ADC driver, to help isolate the problem. Use oscilloscope on the detector output, instead of the ADC_driver+ADC. The goal here would be to eliminate any possible feedback path from detector output back to input. A good low-inductance ground plane for the detector circuit is necessary.
The AD8307 and AD8310 both use internal DC offset cancellation circuitry, but that circuitry is described as guaranteed stable. If this part of the design is suspect, it can be disabled by pulling that pin high or low, as described in the AD8307 datasheet, to force an internal offset of either + or -1.6mV (input-referred). This will reduce input sensitivity, which may or may not be a problem in this application.