[ OPA316 ] General Questions, Gain, Gain Error/Flatness

Hi Ken,

Good questions.  The answers to all of these questions can be found by watching the TI Precision Labs Videos on op amps.

Green:  The open-loop gain of an op amp is typically very high and defines the change in input offset voltage vs. the change in output voltage.  The closed-loop gain error / linearity can be calculated based on the closed loop gain equation:  Acl = Aol / (1+Aol*β).  As the gain term increases, the β term decreases and the gain error increases.  Similarly the gain error increases as the Aol decreases with frequency.

Blue:  This depends how the swing to rail limits are specified in the product datasheet.  This is covered in detail in the 3rd Input Output TI Precision Labs video.   In the datasheet example you provided the swing-to-rail limits are not specified for a particular open-loop gain value.  Therefore the limit shown in the datasheet is a completely saturated output voltage. 

Other datasheets, like the one shown in the slide below, provide a minimum open-loop gain (Aol) specification for the swing-to-rail limits.  In this case the datasheet specification shows the maximum output swing before the open-loop gain begins to degrade.  When the open-loop degrades so does the linearity.  In the OPA827 datasheet example, notice that the swing to rail limits are specified as 3V from the rails to maintain an Aol of at least 120dB (for a 10k load).  If you compare that to the Output Voltage swing vs. Output Current curve (Claw Curve), you'll see that the saturated output voltage can reach well within 2V from the rail but this will be a very non-linear region of operation.