i have avalanche photo diode with which i am trying to digitalize the current from TIA through a comparator, the diode biased using a HVDCDC
the source is being a 20ns laser with a PRF of 3Khz
the circuit is as below
simulate this circuit – Schematic created using CircuitLab
the output of opamp for an high intensity light is like above, we can see the opamp fully saturated, but i am clue less about the over shoot and ringing effect, this happened when i have put my bias voltage 300V, i am not able to stop the opamp to enter in to saturation.
even if i see the datasheet of DC-DC CA05N, it can maximum source 2mA of current, and the peak current of didoe is only 0.25mA, so i understood the dcdc current is exceeding 2mA
so to avoid the opamp going in to saturation i tried to use a 300K in series with the source but still the opamp went to saturation,
it evident that in order to make the opamp not to go in to saturation, the only way i can try is to reduce the bias voltage to as less as possible, so i operated it at 70V for which the response is as below.
reasons are unknow for the expansion of 20ns pulse to 100ns even through the APD is having a BW of 70MHz,
so in order to operate the APD properly, i understood i have to bias with higher voltage to see proper response for low light, but at the same time i have to avoid saturation.
i feel limiting the current to the TIA will solve problem, kindly suggest some ways to bypass the excess current which is to the opamp.
Kindly throw some light on behaviour of APD in saturation also
EDIT:
as the answers suggested to go for a unity gain stable equivalent, i went for changing my opamp to LTC6269 which is 400Mhz unity gain stable
below is the result i got with photo diode polarity inversed, so you may see a inverted output, but effect persists
the analog support was suggesting me also on using a subber at the photo diode bias so i used a 20ohm in series with 100p cap thinking this disturbance is due to the power supply i am using, frankly i am clueless how to tweak a snubber
please help me in identifying the problems rootcause
**EDIT2:**i have to tweak my Cf accordingly i feel, to avoid this, i dont think this is ringing, i feel i have solved my problem , will comeback with results shortly
EDIT3 : replacing Cf with 4pF given me a clean pulse with out any ringing
Thanks for the help
EDIT4: But after few experiments i identified that i am not saturating the photodiode/opamp completely, so i have increase the pulse width and pulse power on photo diode with which my response turned like below
Best Answer
The amplification and reaction speed of an APD depend on the current through it. It's a bad habit of the physics community to use APDs with a voltage source and a series resistor for biasing. As you have witnessed, this fails in high dynamic range applications. I suggest you change your biasing circuit to provide a constant current instead of constant voltage.
The ringing you see is from your opamp. You are using a variant that is not unity gain stable. The -10 in the part number denotes that it's a decompensated opamp for a gain of at least 10 (Linear's opamp numbering convention). But for this kind of circuit to work, you need unity gain stability.
I suggest you have a look at Linear Appnote 92 which discusses a few circuits for APDs.