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

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  • 0
    •  Analog Employees 
    on Nov 6, 2013 8:06 AM over 7 years ago

    Hi Unai,

    Is DC measurement important to you?   If not, you may be able to AC couple in the stage following the transimpedance amplifier.   Then offset error created by the bias current of your transimpedance op amp wouldn't matter as much, since you can remove it in the second stage.

    Do you plan on filtering after the transimpedance amplifier?   This would help with noise somewhat, especially if your transimpedance stage ends up having a considerably more bandwidth than the 4 MHz you mentioned.   Anything you can do to restrict your bandwidth will help your noise.

    A couple of other comments: 

    - Given that your photodiode's capacitance is only 1 pF it's likely the input capacitance of the op amp will dominate

    - Note that the AD8067 has a minimum gain of 8, which may affect your stability.   We are in the process of adding more intelligence to Photodiode tool to handle these cases.

    Matt

Reply
  • 0
    •  Analog Employees 
    on Nov 6, 2013 8:06 AM over 7 years ago

    Hi Unai,

    Is DC measurement important to you?   If not, you may be able to AC couple in the stage following the transimpedance amplifier.   Then offset error created by the bias current of your transimpedance op amp wouldn't matter as much, since you can remove it in the second stage.

    Do you plan on filtering after the transimpedance amplifier?   This would help with noise somewhat, especially if your transimpedance stage ends up having a considerably more bandwidth than the 4 MHz you mentioned.   Anything you can do to restrict your bandwidth will help your noise.

    A couple of other comments: 

    - Given that your photodiode's capacitance is only 1 pF it's likely the input capacitance of the op amp will dominate

    - Note that the AD8067 has a minimum gain of 8, which may affect your stability.   We are in the process of adding more intelligence to Photodiode tool to handle these cases.

    Matt

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