Photodiode amplifier designed by wizard oscillating


I used the Photodiode Circuit Design Wizard ( 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 (, you can load the file included in this ZIP into the wizard:


fixed hyperlinks.
[edited by: HEK at 9:40 PM (GMT -4) on 12 Sep 2018]
  • +1
    •  Analog Employees 
    on Sep 14, 2018 11:34 PM over 2 years ago

    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.



  • Hi Matt,

    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!


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