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OPA211: OPA211 Shutdown Mode Details

FarronDacus
Intellectual 690 points
Part Number: OPA211

Hello: 

I am using the OPA211 low noise op amp in a low noise system where I need to use the shutdown option.  I have two questions:

1.   The datasheet reports that in shutdown the output assumes a high impedance state.  However, since shutdown current is limited to 20uA, it appears that the input op amp stage is also shut down or partially shut down.  So, in shutdown, can voltages within the rails be applied to the op amp inputs without those inputs drawing current beyond the normally tiny input bias currents?  I have some high impedance nodes applied to the inputs that I need to not affect or pull around by the OPA211 being taken in and out of shutdown. 

2.  I could check this myself if there was a TINA TI op amp model for the OPA211 options (the DFN-8 and MSOP-8 package forms) that have shutdown.  But, in the TINA TI "SPICE Macros => Op Amps" listing there are no versions of the OPA211.  Nor do I find any TINA downloadable models on the TI website, though there is mention of PSPICE models.  Is there a TINA TI SPICE model of the OPA211 with shutdown, or does that have to be simulated in PSPICE?

Thanks,

Farron

  • 0
    TI__Guru* 93105 points

    Hello Farron,

    When the OPA211 is switched from the enabled state to shutdown the supply current drops from the typical operating current of 3.6 mA, to a few microamps. Not only does the output stage shut off, but pretty well everything the rest of the op amp including the input stage shuts down too. The input stage is not biased in its linear operating state and any applied voltage can result in substantial input currents flowing. In the past, I happened to have measured the OPA211 input currents when the op amp is in disabled state. As can be seen in the attached graph the input current can be quite significant as the input voltage is moves further away from 0 V. So indeed the OPA211 would pull down your circuit's high impedance nodes, which is not in line with your application's requirement.

    The OPA211 simulation model, be it the one for TINA Spice or Pspice for TI, does not model the Enable/shutdown pin function. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering