I used the Photodiode Circuit Design Wizard (http://www.analog.com/designtools/en/photodiode/) to come up with the attached circuit.
The circuit simulation opamp oscillates on the output. I guess that chances are that it will do so in the real world as well.
Please could someone advise; why and what to do about it?
I attached a ZIP which includes 2 files;
1. TransientAnalysis.asc (generated by the wizard)
2. TransientAnalysis_delay.asc (modified by me; i simply delay the pulse from the photodiode by 900ns) which shows oscillation after approx. 200 ns.
If you want to check out the design in the wizard (http://www.analog.com/designtools/en/photodiode/), you can load the file included in this ZIP into the wizard:
OK figured out what is going on here. The LTC6268-10 is a G=10 stable amplifier and the circuit is violating that.
In a photodiode circuit you have the feedback capacitance (Cf), the photodiode capacitance (Cd), and the op amp input capacitance (Coa) For stability purposes the noise gain of the circuit is (Cd + Coa + Cf) / Cf.
The LTC6268-10 is a G=10 stable amplifier. This means (Cd + Coa + Cf) / Cf should be 10 or greater. There may be a little bit of play where you can push it under this number, but I think this is a pretty good rule of thumb. Normally Cd >> Cf and this is not an issue, but for this particular circuit, Cd and Cf are about the same magnitude, giving a noise gain of about 2, which makes the circuit unstable.
There are two lessons here for us at ADI regarding our photodiode wizard:
1) The photodiode tool should be smart enough to prevent you from getting into this situation in the first place
2) Our model of the LTC6268-10 poles and zeroes in the wizard could be improved so that if this were to happen, we'd see the ringing in the time domain plot.
I'll file a ticket for us to work on these things.
In the meantime, I think it will be difficult to find an optimal feedback cap for the LTC6268-10 with a 402 ohm feedback resistor and a Cd of 5pF. Something like the attached might work, where the extra 80 ohm resistor helps to increase the circuit's total noise gain. The LTC6268-10 datasheet apps section also has some commentary on min and max Cf tradeoffs.
thank you for your explanation!
I understand and see that you needed to lower Cf in order to achieve stability.
I will try to be careful with the layout here and follow the layout recommendations of the LTC6268-10 datasheet to minimize spurious Cf capacitance, and I will add a resistor to GND on negative input as well and cross my fingers!