photodiode amplifier and bias current

Hello,

I have to measure the current of a photodiode.

Input signal is:

  • Maximum current 15uA.
  • Frequency of the pulsed current 4MHz
  • Photodiode capacitance: 1pF

I have done the desing using the AD8067.

I have obteined good results, but i need to improve the SNR. The source of noise is the input voltage noise, so i have been looking for opamps with lower values. The AD8067 has 6.6nV/√Hz, and i have found the ADA4897-1 which it has only 1nV/√Hz. The problem is that the maximum input bias current is -17uA. How does it affect? is there any way of overriding it?

In the analog photodiode wizard (http://www.analog.com/designtools/en/photodiode/#/photoDiode) one of the recommended amplifiers is the AD8048, which it has 3.8nV/√Hz but 3.5uA of input bias current.

Thanks

Unai Goyarrola

Parents
  • Hi Matt,

    I don't care about the DC measurement.

    I am a little bit confused about the bias current input concept. I have always thought about input bias current as a current that it comes/go from the input. What it means, in my case, that the photodiode must supply this current, i am wrong? So my doubt is: if my photodiode output is 15uA, and the input current bias of the ADA4897-1 is 17uA, what will be the current flowing through the gain resistor?

    Thanks for your help,


    Unai Goyarrola

Reply
  • Hi Matt,

    I don't care about the DC measurement.

    I am a little bit confused about the bias current input concept. I have always thought about input bias current as a current that it comes/go from the input. What it means, in my case, that the photodiode must supply this current, i am wrong? So my doubt is: if my photodiode output is 15uA, and the input current bias of the ADA4897-1 is 17uA, what will be the current flowing through the gain resistor?

    Thanks for your help,


    Unai Goyarrola

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