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OPA2210: Differential input impedance, datasheet vs. PSPICE model

Ue
Genius 16565 points
Part Number: OPA2210
Other Parts Discussed in Thread: OPA211

Hi all

My customer is using the OPA2210 for voltage monitoring configured as differential amplifier with an upfront resistor divider and they have this question:

In the datasheet they find for the differential input impedance:

But doubt that 400kOhm is the correct value as this seems to be very low.

In the PSPICE model they find:

R_R_DIFF       ESDN ESDP R_NOISELESS 100MEG

Their comment is that even this looks rather low.

Can you comment as to the value for the differential input impedance they should expect ?

Best regards

Ueli

  • 0
    TI__Guru 54426 points

    HI Ueli,

    As you have mentioned, in the difference amplifier application, the device is configured with external resistors, where the differential amplifier effective input impedance is defined by the external R1, R3  input resistors at the op-amp inverting and non-inverting terminals.  The actual difference amplifier differential impedance is given by R1+R3 external resistors as seen on the figure below.

    The op-amp differential input impedance is a little bit of a confusing spec as that is the small-signal impedance with no feedback.  The input impedance into the input of the amplifier during normal operation, is actually very high, as only the input bias current flows at the input terminals of the amplifier while the amplifier is within the linear region, and this impedances are larger due to the feedback loop-gain.  Also, while the amplifier is active in it's linear region, the input terminals of the op-amp are a "virtual" short with only a small offset between them. 

    It is important to highlight, the input terminals of the OPA211 are protected from excessive differential voltage with anti-parallel diodes, as shown in Figure 44. 

    Regarding the input impedance parameter on the PSPICE model, I will need to verify with the Model engineer and get back to you.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Guru 63890 points

    Adding to Luis' answer, if you look at the small-signal model of BJT, you will notice Rπ=β*re between the base (B) and emitter (E) - see below.

    Thus, under normal operation where input terminals are at virtual short, the OPA2210 differential input impedance is simply:

    Rin_diff = 2*Rπ = 2*β*re = ~400kohm

  • +1 in reply to Marek Lis
    TI__Guru 54426 points

    HI Marek,

    What is the effect of the op-amp feedback loop gain (AOL*Beta) on the effective input impedance into the op-amp input terminals seen on a closed-loop circuit?  Does the loop-gain increase the closed-loop input impedance looking into the non-inverting terminal of the op-amp when in a closed-loop configuration?

    I think that the closed-loop small-signal input impedance looking into the non-inverting terminal of the op-amp is very high, around the order of (1+T)*2*Rπ, where T is the loop-gain (AOL*Beta).

    In the overall difference amplifier circuit, the user will still see the R1+R3 effective input impedance by the external R1,R3 resistors; and a virtual short between the inputs.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Genius 16565 points

    Hi Marek, hi Luis

    Thank you for the quick reply. I will ask the customer to provide / post the schematic to help us understand what the concerns are.

    Best regards

    Ueli

  • 0 in reply to Luis Chioye
    TI__Guru 63890 points

    Hi Luis.

    As you correctly said Rin_diff of 400k specified in the OPA2210 datasheet is part of a small-signal input impedance that for large Vin_diff input signal should be neglected in close-loop difference amplifier configuration because the feedback keeps Vdiff~0 (see below).  Thus, the effective large-signal input resistance seen by Vin_diff is just the sum of R1 and R3. 

  • +1 in reply to Marek Lis
    TI__Guru 54426 points

    Hi  Ueli,

    I have not heard back, therefore I will close this post.  Nevertheless, please feel free to post if you have further questions.

    Thank you and Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Genius 16565 points

    Hi Luis

    I didn't hear back from the customer so yes, this post can be closed.

    Best regards

    Ueli