the open loop output impedance of the OPA2320 is 90 Ohms at 1 MHz. In the data sheet, there is also a curve that shows Output Voltage Swing versus Output current. Any give point on the curve indicates a much lower Zo than the 90 Ohms specified. For example, at 40 mA output current, the output voltage is 2V while the rail voltage is 2.75V. The 0.75V drop at 40 mA indicates an output impedance of 18.75 Ohms.
Why is there such a discrepancy from the 90 Ohm measurement? At what gain, frequency, etc. was the voltage vs.current plot taken?
Thanks for any insight you can offer.