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Input impedance simulation of Op Amp circuits

Alberto Aloisio
Intellectual 900 points
Other Parts Discussed in Thread: TINA-TI, OPA1611

Dear Staff,

I am tryin to simulate the input impedance of the circuit show in the TINA-TI design:

filter_OPA1161.TSC

I am following the method shown in this thread:

   As you can see, the input impedance goes down very fast, about 10 MOhm at 10 kHz.

Is such a result correct ? I am doing something wrong ?

Thank you for your help and assistance.

Kind Regards,

Alberto

  • 0
    TI__Expert 6400 points

    Hi Albert,

    I will do double-check the validity of simulation model to our team.

    While, your simulation looks no problem, especially you use the improved TINA-spice model.
    As you can see in the macro model of OPA1611, this model was updated in 7th Feb. 2019 and certified as "GWL"  Op Amp Macro-model.
    You can see the details of GWL Op Amp Macro-model in the below application note.
    http://www.ti.com/lit/an/sboa338/sboa338.pdf
    Furthermore, according to the preamble of OPA1611 TINA-spice macro model, the both the differential and common-mode input impedance should be simulated properly.
    I will attach a part of the preamble of OPA1611 TINA-spice macro model in as below.

    *************************************************************************************************
    *
    ** Released by: Online Design Tools, Texas Instruments Inc.
    * Part: OPA1611
    * Date: 07FEB2019
    * Model Type: Generic (suitable for all analysis types)
    * EVM Order Number: N/A
    * EVM Users Guide: N/A
    * Datasheet: SBOS450C -JULY 2009-REVISED AUGUST 2014
    * Created with Green-Williams-Lis Op Amp Macro-model Architecture
    *
    * Model Version: Final 1.2
    *
    *****************************************************************************
    *
    * Updates:
    *
    * Final 1.2
    * VOS drift feature is added
    * Added Unique subckt name, removed Claw ABS.
    *
    * Final 1.1
    * Release to Web.
    *
    ****************************************************************************
    * Model Usage Notes:
    * 1. The following parameters are modeled:
    * OPEN-LOOP GAIN AND PHASE VS. FREQUENCY WITH RL, CL EFFECTS (Aol)
    * UNITY GAIN BANDWIDTH (GBW)
    * INPUT COMMON-MODE REJECTION RATIO VS. FREQUENCY (CMRR)
    * POWER SUPPLY REJECTION RATIO VS. FREQUENCY (PSRR)
    * DIFFERENTIAL INPUT IMPEDANCE (Zid)
    * COMMON-MODE INPUT IMPEDANCE (Zic)
    * OPEN-LOOP OUTPUT IMPEDANCE VS. FREQUENCY (Zo)
    * OUTPUT CURRENT THROUGH THE SUPPLY (Iout)
    * INPUT VOLTAGE NOISE DENSITY VS. FREQUENCY (en)
    * INPUT CURRENT NOISE DENSITY VS. FREQUENCY (in)
    * OUTPUT VOLTAGE SWING vs. OUTPUT CURRENT (Vo)
    * SHORT-CIRCUIT OUTPUT CURRENT (Isc)
    * QUIESCENT CURRENT (Iq)
    * SETTLING TIME VS. CAPACITIVE LOAD (ts)
    * SLEW RATE (SR)
    * SMALL SIGNAL OVERSHOOT VS. CAPACITIVE LOAD
    * LARGE SIGNAL RESPONSE
    * OVERLOAD RECOVERY TIME (tor)
    * INPUT BIAS CURRENT (Ib)
    * INPUT OFFSET CURRENT (Ios)
    * INPUT OFFSET VOLTAGE (Vos)
    * INPUT OFFSET VOLTAGE VS. TEMPERATURE (Vos Drift)
    * INPUT COMMON-MODE VOLTAGE RANGE (Vcm)
    * INPUT OFFSET VOLTAGE VS. INPUT COMMON-MODE VOLTAGE (Vos vs. Vcm)
    * INPUT/OUTPUT ESD CELLS (ESDin, ESDout)
    ******************************************************

    Thank you and Best regards,
    Iwata Etsuji

  • 0 in reply to Iwata Etsuji
    TI__Expert 6400 points

    Hi Alterto,

    I have checked this with a TINA simulation.
    Could you please check the below simulation file?

    OPA1611Zin.TSC

    The modeled input impedance of OPA1611 is comparable to the input impedance specifications in the OPA1611 data sheet.

    Thank you and Best regards,
    Iwata Etsuji

  • 0 in reply to Iwata Etsuji
    Intellectual 900 points

    Dear Iwata-san,

    thank you for your reply.

    Can I conclude that my original simulations are indeed correct ?

    Thank you again for your time.

    Regards,

    Alberto

  • 0 in reply to Alberto Aloisio
    TI__Prodigy 715 points

    Hi Alberto,

    Your results seem reasonable.  Let's think of it another way...  The OPA1611 shows a typical common mode capacitance of 2pF.  At 10kHz, the common mode impedance will be Zc = 1 / (2*pi*f*C) = 1 / (2*3.14*10000*2e-12) = 7.9Mohm.  The common mode input capacitance of only 2pF is actually quite low and for many op-amps the common mode impedance would be much lower at 10kHz.   

    Thanks,
    Scott 

  • 0 in reply to scottgulas
    Intellectual 900 points

    Dear Scott,

    thank you for your comment.

    I guess, we can close this case.

    Kind Regards,

    Alberto