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TINA/Spice/OPA847: some questions about the Aol Gain

xin xie
Prodigy 60 points
Part Number: OPA847
Other Parts Discussed in Thread: TINA-TI

Tool/software: TINA-TI or Spice Models

Dear TI :

From the datasheet,We can consider the open-loop gain Aol of OPA as a product of  a DC gain K and a Pole,so could we say  Aol=K/(1+τs)

then the close loop gain of a TIA could be expressed as AOL/（1+AOL*β）

β is the feedback factor

from the datasheet, I estimate the K and τ, β is come from the feedback net. then I put them into AOL/（1+AOL*β）,and get the bode plot of  the close loop gain of TIA 

but 

the bode plot has a lot of differences with the result of  TINA simulation.especially the frequency of peak.there are two Pole and a zero in  the close loop gain I got .the real-world test result is same with the TINA simulation.

so I want to ask, where do I make a mistake, I have a confidence on β, maybe the Aol or the close loop gain has a mistake,could I get the Aol equation  from spice model of TINA  ?

please give me some advise

here is the schematic and result of TINA simulation.

  • 0
    TI__Genius 9775 points

    Xin,

    You can start by extracting the open loop bode plot of the model. This can be done by doing simple open loop testing to the opamp/model to see if it matches your expectation in term of the transfer function. Then from that bode plot, you can try to find the appropriate equation (by curve fitting) to see if your equation can match the open loop bode plot.

    During your open loop testing, make sure that you offset the input with appropriate dc to cancel the input offset voltage. So if the parts offset say 3mV, then at one of the input you have put either +/-3mV to cancel the input offset such that you can position the output voltage at around middle rail of the power supply. Without doing this step, you will cause the output voltage to rail out at around the power supplies. Once this is done, make sure also that you load the output with the appropriate loading (resistor and capacitor) for this open loop test to the desired loading you have for your particular test/situation.

    FYI, usually opamp open loop is characterized by at least 3 poles system where the first pole is the low frequency dominant pole, the second pole is higher frequency dominant pole, and finally the third pole is higher frequency non-dominant pole to bring the phase of the open loop plot great than -180 degree.

    Herman