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OPA277 issue with AC analysis

cedar kuoch
Prodigy 10 points


i am trying to model a photo detector and in the process i am model a photo diode. The input signal i am trying to model is 1 mW DC and 1 microW AC. I biased the input to 1 V and 1 mA DC . I have it going to a power meter that outputs 1 mW, amp meter that outputs 1 mA and a volt meter that outputs 1 V.

The problem is when i set an AC current of 1 microA in the biased (1 V) current source, in the "table of AC results," i still get 1 mW and not 1 microW. I am also reading 1 mA from an amp meter.

I have the current source set to 1 microA, 10MHz, and 0 deg phase sine wave.

Any help would be appreciated because i am new to this. 

Thank you in advance.

photodiode TIA Noise.TSC
  • 0
    TI__Genius 12335 points

    Cedar,

    The output of your op amp is saturated at the positive rail, giving erroneous results for the simulation. For 1mA DC, the output voltage of the op amp will be 1mA * 10000 = 10V , however since the power supply rails you have provided are +/-5V the op amp output will go as far as possible to the positive rail and saturate at this voltage. This is also why the output of the op amp will not respond to an input AC signal. A quick search of e2e will turn up many threads on how to AC couple photodiodes to prevent the dark current portion from saturating the output. For example here is one thread: http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/300533.aspx