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OPA858: Need help with the simulation for this device.

Houari Cobas Gomez
Prodigy 90 points
Part Number: OPA858

Tool/software:

Hi, I´m doing simulations with the OPA858 device  and I am getting some peaks in the frequency responce that I was not expecting. Then I moved to do  some basic simulation to try to obtain and verify the operational parameters and there is some variations in the frequency response that i was not expecting. 

I´m trying to replicate the simulations idea of the document "What You Need to Know about Transimpedance Amplifiers – Part 1" at page 2 for determining the magnitude and phase of loop gain, AOL and 1/β. I was expecting a  curve  like this one stated iin  the document

but I rather obtained   this one 

see that close to the 1GHz there is something in the magnitude response that introduce som kind of zero and then a pole. The phase for the Aol also suffer some perturrbation at the same frequenccy.

the circuit I´m simulatin is this one 

The model I´m using I extracted from the opa858 website andd Im simulating in altium. 

I defined the Aol in the lower circuit as Vo/V-2 and the B in the uppere circuit as V-/V02  and then calculatedd th e1/B

When I change the R1 value fromm 100K to 0.0001 ohm I obtained this result

Can you help understand why Am I having this result?

The problem started  because I was simulating this circuit

and  I´m obtainning this result

here I[m trying to get bacck the R8 resistor value using the equation (v(VDUT+)-v(VDUT-))/((v(VDUT-)-v(OUT))/100Kohm)

for the low frequencies I obtain the correct value, but then there iis this peak at the 25.14 MHz that i was not expecting and  need to understand what is happening for removing this peak.

Thanks you very much

  • 0
    TI__Mastermind 28915 points

    Hello Houari,

      We apologize for missing your thread earlier.

      Thanks for the detailed post, and yes you are correct stability simulations should look similar to the results from the first figure.

       Here is a detailed e2e thread on doing stability simulations: OPA855 detailed example TIA.

       For your specific initial circuit, I recreated it below when breaking the loop at output: 

       Here it is breaking the loop at the input:

       In above case, this circuit will not be stable due to loop gain phase < 45 degrees. This is because there is no input/feedback capacitance included to shape the noise gain for >7V/V for stability for this device. In TIAs, they naturally have high closed-loop gain and noise gain: 1 + CInputTOT/CF. 

       It looks like you were breaking the loop at the input, but would still need to include the input capacitance of the amplifier (due to breaking the loop at the input), and did not include the feedback resistor.

       For your actual circuit, will this be a input current source from a PD, or will be a regular voltage source and will need to switch gains depending on the voltage source output?

    Thank you,

    Sima