Hi,
I am using the LMH6624MA with an ADP, which is biased at 500V, can you please suggest a low capacitacnce protection method in case the 500V ends up on the input? i was considering a singal diode (BAV99) but they are only rated to 100V.
Thanks
Ok, here I reduced your bias to 20V to just get things going, you probably already have them, but a couple of caps will help your noise, That noise peaking is very normal and dependent on the source C
The BAV99's add a little bit of C. If the APD shorts out, the 10kohm will limit the current to 50mA, well in range for the BAV99. Their voltage with 50mA might increase beyond the internal input stage protect diodes (if any) that would conduct back to the V+ node., -That current split would be limited by the 5kohm on the V+ input to ground. So, with 50mA into the BAV99, only say about 0.7V on the V- node -overdriven to the output but not beyond the supplies - I would think fine. and the file,
As long as I have the file, lets look at the 2nd stage where the feedback cap risks instability as it is shaping the noise gain to 1 for this decomp part. Usually easier to see in the output spot noise, yep - looks kind of chancy but easy to improve,
We can shape the noise gain to stay higher with another cap on the inverting input, hold flat noise gain (you will still get the gain filtering).
yes, this is a lot better phase margin with the noise peaking with a broad peak,
And the signal response curve,
More than you would ever want to know about this inverting comp thing I published way back in the BurrBrown days. This was recently republished.
https://www.edn.com/unique-compensation-technique-tames-high-bandwidth-voltage-feedback-op-amps-2/
Well Andrew,
It is kind of encouraging that the LMH6624 model basically predicted that oscillation - little mismatched on frequency probably due to not having the correct loading in sim vs bench. You have been showing only a 75 series out - a lot of this higher speed part will move towards less phase margin with no load so I have been adding a terminating 75 in sim.
The LM7171 is kind of an odd choice, did you have stock in it or something?
It falls into a class of VFA that has a high tranconductance input stage to offer CFA type slew rate (the OPA690 is the part we did at BurrBrown like this) - that comes with much higher input voltage noise,
Your source in the model is not correct for an APD (even though they often specify a 50ohm out, that is for their characterization I think - Hammamatsu I assume)
Anyway, a similar GBP lower noise 2nd stage might be the OPA820. Here is an output noise comparison,
And this file,
Well I did the OPA656 as well, unity gain stable JFET input which is not needed in your design. Relatively pricey to get the JFET input device.
MSL3 is kind of odd, I had standardized on MSL2 with the thinking the MSL1 allows a customer to completely abuse a part in storage and the company will back that up.
The National guys in that late 90s timeframe where pretty aggressive in a number of ways. MSL1 might be one of those, the LM7171 was going after similar parts down the road at LTC - the LTC families of that type were kind of overdone - they got real enamoured with that topology as they had kind of missed the boat on CFA.
Yes the SO-8 OPA656 is level 3, but the sot23 is level 2. Maybe there was some actual problem in those days getting MSL2 for the SO-8. I don't think that continued over time. I think maybe the low leakage input bias current for the JFET might have perhaps failed MSL2 testing in the SO8?
Yes, the OPA820 is more like what I recalled, all packages MSL2
