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OPA690: Output impedance

Marvin Gasis Arcangel
Genius 9635 points
Part Number: OPA690

Hi Team,

The data sheet says the output is high impedance when its disabled but in simulation is appears to have a low resistance load. Feeding a 1V DC source back to the output of the amp through 25 ohms it appears the amp output is ~18 ohms. It may be the spice model is not set up for such a simulation. would you kindly confirm?

Here si the .TSC file for your reference:

OPA690 Disabled Simple.TSC

Regards,

Marvin

  • 0
    Guru 48205 points

    Unity gain, very good chance you are turning on the input stage at the V- input, that then biases up the current mirrors turning on the output stage, you might try this at a gain of 10 say. If you need unity gain try back to back Schottky diodes across the inputs - low C of course. 

  • 0 in reply to Michael Steffes
    TI__Genius 14141 points

    Hello Marvin,

    I am working on a TINA sim for you; you might try replicating the circuit present in Figure 23 of the OPA690 datasheet; you can use a current source instead of a voltage source to remove an extra step of the process.  By using a current source injected into the amplifier, the transfer function (being output over input) would resolve as the output (voltage measured at OUT pin) divided by the input (current injected into amplifier).  This cleanly displays as Ohm's Law, where the impedance is V/I.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 14141 points

    Hello Marvin,

    I have confirmed by simulation the model for the OPA690 does not model output impedance over frequency; you would want to look at output impedance through an AC response, as output impedance changes with frequency.  The OPA690 model does not support changing output impedance over frequency, please refer to Figure 23 for information to that end.

    Typically DISABLE functionality is not set up to model the high impedance state of the amplifier; in many models we suggest customers simulate the 'active' mode and use the datasheet to determine functionality when disabled or power-down is active.

    Best,

    Alec