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OPA316: Use of resistor in parallel with the input signal ; Op-Amp

TapasXplore
Expert 1280 points
Part Number: OPA316
Other Parts Discussed in Thread: TIPD204,

Hi,

While referring to this design (TIPD204, refer fig.1 pasted below), I was not able to understand, why 49.9 Ohm (RTERM) resistor is being used in parallel with the input signal. Is it to do with increasing the input impedance of the op-amp. Op-amps already have very high input impedance. Pl help me in understanding it. 

Fig. 1

  • 0
    Guru 48195 points

    This is very poorly drawn Tapas, 

    Normally, Rterm is "termination"resistor for a 50ohm measurement system using a network analyzer - a more accurate drawing would include a 50ohm series source resistor if this was a test set up. 

    Sometimes a resistor to ground is used to make sure there is a bias current path if the source is opened or AC coupled. I would suspect you can eliminate it if the source is DC coupled, No supplies shown, but be careful of output headroom issues. If this is single supply, usually a 0V input drives the output into a saturated rail condition. 

  • 0
    Guru 140200 points

    Hi Tapas,

    as this is no HF circuit and the OPA316 is no HF OPAmp either, I think it's a simple drawing mistake. Eventually, the 50R resistance is a leftover from the frequency response measurement which might have been carried out with 50R equipement? But in your application you can easily increase R4 :-)

    Kai

  • Former Member
    0 Former Member

    Hello Tapas,

    I have reached out to both of the co-authors to see whether they can recall why this was done.

    I will post an update as soon as I have one, but will check in regardless early next week.

    Regards,
    Daniel

  • Former Member
    0 Former Member in reply to Former Member

    Hi Tapas,

    I have confirmed that the resistor is for measurement purposes.  From one of the co-authors:

    It’s a 50Ω termination resistor (RTERM) for use when measurement equipment drives the inputs.  The Bode100 we used to test requires a 50Ω termination for best performance.

    Let us know if you have further questions.

    Regards,
    Daniel