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LMC6484: Output Resistance

Kiran Kerai
Intellectual 380 points
Part Number: LMC6484

It appears from testing our design that the output resistance of the LMC6484 opamp varies when the opamp reaches saturation.

In our application we have configured the opamp as a follower and have 270 ohm seried resistor connected to the output to a successive approximation ADC. As the opamp reaches saturation the voltage at the output of the opamp (measured by ADC) is non-linear (lagging).

We are using a MAX1230 ADC from Maxim that specified input resistance should be limited to <300 ohms and I suspect that the opamp output resistance is exceeding this resulting in non-linearity on measurement. <300 ohms is required because the ADC contains a capacitor on the input that must be charged during TRACK/HOLD cycle so that the input voltage is SAMPLED and measured correctly.

The LMC6484 opamp specifies approx. 110 ohms output resistance at Vs=5V. Can you confirm that the effect we are seeing is a result of increase in output resistance as the opamp saturates?

We are seeing non-linearity between 4.7 and 4.8V which recovers beyond 4.9V. Can you confirm that this change in output resistance will only happen in this range, close to saturation?

  • 0
    TI__Intellectual 795 points

    Hello Kiran, 

    There will be non-linearity on the output of LMC6484 as its value approaches the positive supply voltage. In other words, the op-amp linear output range will be limited relative to the positive supply by a few hundred mV, depending on the output load resistance and temperature. This is due to the output voltage swing limitations; electrical characteristics table on page 6 of the data sheet shows the output swing characteristics.

    https://www.ti.com/lit/ds/symlink/lmc6484.pdf

    So, the behavior of the op-amp (as measured by the SAR ADC) might be due to the op-amp itself. Of course, that assumption holds as long as the op-amp is powered using a 5V single supply.If the LMC6484 supply voltage is higher than ~5.5 V, there should be no non-linearity in the 4.7-4.8 V range, and this might indeed come from the SAR ADC. 

    As for the output resistance of LMC6484, the electrical characteristics tables do not specify this parameter. It is given in the 'Feature Description' section of the data sheet, and only as an approximation. The data sheet doesn't define or specify the change of this parameter over voltage or temperature, so I wouldn't be able to accurately answer your question, Kiran. 

    I hope this helps. Please let me know to what supply voltage is used on LMC6484. 

    Regards,

    Vladimir