OPA177: Offset levels

Hello Vidhya,

The OPA177 input common voltage range (Vcm) has a lower limit +2 V above the V- supply voltage being applied. Therefore, with V- being 0 V in your schematic the minimum Vcm would be +2 V. Thus, setting Vcm at 0 V would be -2 V below the linear common-mode operating range. The behavior you find from the TINA-TI first simulation is indication of an OPA177 Vcm range violation. By time the Vcm input voltage is moved to +3 V, one volt above the Vcm minimum of +2 V as in the second simulation, the op amp performance is as expected.

Be aware that no op amp operating from a single positive supply will have an output voltage (Vo) swing all the way down to 0V. It may swing very close to 0 V, but because of the finite resistance of the output transistor it won't swing exactly to 0 V.

The OPA177 is legacy op amp and shouldn't be considered for a new design. I suggest having a look at the OPA192, or OPA197 as modern, high-performance replacements. Their Vcm lower range limit is -100 mV below 0 V so there won't be a Vcm range issue with them. Their output range is within a few tens of millivolts of the supply rails depending on the output current being demanded by the load. The lighter the load, the closer the output approaches the 0 V and 15 V supply rails in your circuit. Have a look at the OPA197 datasheet:

https://www.ti.com/lit/ds/symlink/opa197.pdf

Like the OPA177 the OPA197 is available in the SOIC (8) package. It doesn't have the offset control pin functions of the OPA177, but that really is an obsolete feature not utilized in almost any application today.

Regards, Thomas

Precision Amplifiers Applications Engineering