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DDC112: Photodiode dark current

BP
Prodigy 140 points
Part Number: DDC112

Hello,

I have connected photodiode directly to the input pin of the DDC112 to avoid board leakage effects.
The selected DDC112 input range is -1.4 to 350 pC. with continuous mode conversion.

When no light falls on the photodiode (covered completely) the data output is approx  -0.057pC.

Is it due to the dark current of photodiode, which gets reversed biased due to the input circuit of the DDC112 ?

Should we add or subtract the absolute value of this dark current to our normal signal output when the photodiode is exposed to light ?

Because at low light intensity levels this dark current makes significant difference.

Please help me understand this.

  • 0
    TI__Expert 4680 points

    Hello BP,

    You did the calculations right... So, yes, it seems that you have some current flowing out of the DDC. Don't know what your integration time is, but this offset sounds quite big/not normal. Per the DS, the input DC bias can be 2mV worst case. So, maybe you can check what the PD dark current would be on that case... You can also disconnect the PD to see if the result still the same...

    But I agree that if this is in the end real, then yes, the normal approach would be to subtract it from future measurements. Not an ideal case, unless you do calibrate it out often enough, though, as it may change with temp.

    Regards,
    Edu

  • 0 in reply to Eduardo Bartolome
    Prodigy 140 points

    Thanks Edu

    Integration time is approx 2ms. But at different integration times also the value is close to -0.057pC.

    PD Dark current is 50pA at VR = 10mV. Dark current below 10mV reverse voltage is not given in the datasheet.

    PD shunt resistance is 0.6 GOhm at VR = 10mV

    Is this dark current not normal ? or there may be wrong calculations from my side.

    What should be dark current according to your calculations.

    What could be the reason causing this ?

  • 0 in reply to BP
    TI__Expert 4680 points

    That is interesting... If you got some leakage at the input, usually it should show up as output changing respect to different integration times but you seem to say it is not the case. Like if you go to 4ms, do you still get the same value?

    Dark current wise, 0.057pC in 2ms --> 28.5pA (just divide one by the other)

    But I am not familiar with typical dark currents. I believe those will change quite a bit from PD to PD. So, can't comment on that. But you could measure the voltage at the input of the DDC which I anticipate should be very small, so, yes, that dark current sounds big...

    Have you tried disconnecting the input? Newer devices have a mode where the input can be disconnected. Unfortunately this device doesn't... so you'll have to physically do it. Or you can use the TEST mode which injects 13pC packets at the input but not sure about their accuracy (so, not sure you can tell 57fC from those).

    You could also try a different PD.

    Also, are both inputs giving you the same?

    Regards,
    Edu