Part Number: OPA858
Other Parts Discussed in Thread: OPA855
Hello,
We are currently using the OPA858 in a simple trans impedance amplifier circuit as below. The photodiode sees photons of scattered laser light and we use the OPA 858 as a fairly standard TIA as below. The system operates nominally (we are able to run at high gains, clipping the max positive level) for a period of time, however after some time (hours) the amplifier seems to exhibit ringing behavior (but still sort of functions). The ringing behavior seems to develop gradually (after some hours of service) and at first the ringing is intermittent, however as time goes on it seems to become more permanent (suggesting possible gradual damage to the amplifier).
Powering off the amplifier (when it is ringing) and lowering the laser power and powering it on results in clean operation, however as soon as (the aged chip) sees a clipped signal - it will start ringing very quickly. New amplifiers are able to see a clipped signal and operate nominally for some hours (leading us to suspect some gradual damage is being possibly done)
For maximum sensitivity reasons - it is desirable in our application to run with the laser power set as high as possible and the Rf gain set as high as possible. In this case - the amplifier is nearly always "clipping" the positive pulse (the pulse output fully runs to the max level as the photo diode receives a lot of light during its pulse).
This issue has been replicated on two separate boards and replacing the OPA858 on the PCB with a new one seems to make this issue go away for a while.
In order to check there was ample margin - adding pF amounts of Cf (in parallel to the Rf feedback resistor) does not make the ringing go away - but does of course limit the system bandwidth in an undesirable way.
Is this amplifier particularly sensitive to being overdriven (I.e so much light received at the PD that the output basically looks like a positive going square wave that hits the rail)?
Do you have any suggestions for how we may further isolate the problem?
Are there ways you suggest protecting the amplifier (if this is indeed the problem) so that we can operate it in this mode, or an alternative part that may not face this issue?
EDIT : Tried this with an OPA855 and still seeing similar behavior (lowered the gain) - it may not be damage but trying to figure out what causes the amp to become unstable .
