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OPA209A Inverting Input

Hong Tay
Expert 2465 points
Other Parts Discussed in Thread: OPA209, TS321, OPA314

Hi Sir,

My customer is designing using OPA209A. They have concern on the OPA209A during their testing. Inverting input voltage level (IN-) in our circuit configuration below is not what we expected with the OPA209 op-amp. We expect it follows the voltage level across the non-inverting input (IN+) down to zero.

 

By simulation (as attached picture), OPA209A indeed behaves as it is in our testing. We tried to simulate as well another op-amp TS321, and this behavior is what we wanted.

Anything you suspect going wrong? Anything can I ask my customer for cross check?

Hope to hear from you soon. Thank you very much.

  • 0
    Expert 2465 points
    Hi Sir,

    Any recommendation?

    Thank you very much.
  • 0
    TI__Guru* 93105 points

    Hello Hong,

    The issue that is occurring in your circuit is due to the common-mode voltage range (VCM) of the OPA209, which is quite different than that of the TS321. The OPA209 VCM is rated as (V-) + 1.5 V to (V+) - 1.5 V, which is +1.5 V to +3.5 V when V+ is set to +5 V. Therefore, the input voltage applied to the non-inverting input has to be within +1.5 V to +3.5 V for the OPA209 to be operated within its linear operating range. If the voltage is not within the range, then normal op amp behavior won't necessarily be observed. That is whay the inputs voltage levels are moving apart.

    The TS321 is has a VCM range rated from 0 V to (V+) - 1.5 V, which for a +5 V supply is 0 to +3.5 V. Thus, when the input is at 0 V and all the way up to +3.5 V it is operating within its linear range. Since your circuit is operating from a single, +5 V supply an op amp that is rated for rail-to-rail input, and output, would provide the widest input voltage range from a VCM standpoint.

    I ran some simulations of your circuit using the OPA209 and one of our RIRO op amps, the OPA314. The voltage at each op amp input is is measured as the input voltage source is swept from 0 to +3.3 V. You can see the differences in the input voltage levels for each the two op amps.

    Regards, Thomas

    PA - Linear Applicaitons Engineering

     

  • 0 in reply to Thomas Kuehl
    Expert 2465 points
    Hi Thomas,

    Really appreciate your detail explanation.

    Thank you very much for the support.