Hi,
My customer uses OPA548 with a 220nF capacitive load. Will that be ok? What is the max capacitive load?
The OPA548 is configured in a buffer configuration, DC input, but for some reason it begins to swing. Any idea, why it starts to swing?
Thanks
Christian
Hello Christian,
If the OPA548 is in a buffer configuration (Av = +1 V/V) and driving a purely capacitive load of 220 nF, then it is likely that the op amp will oscillate due to insufficient phase margin.
The OPA548 datasheet in the Figure 19 graph depicts the Small-Signal Overshoot vs Load Capacitance behavior. I show the graph here:
Note the curve for G = +1 V/V tops out at about 12000 pF (12 nF) and has a corresponding 50 % small-signal overshoot. Applying the Phase margin vs. Percentage Overshoot graph in TI's Analog Engineer's Pocket Reference, 50 % overshoot corresponds to a phase margin of about 25 degrees. You really wouldn't want to apply a circuit with a phase margin lower than that for stability and transient response reasons. 220 nF is almost 20x the capacitance of 12 nF so you can see that would be an issue.
If there is any other output load in parallel with the capacitor that could change things somewhat, but the stability would need to be analyzed. If you have the complete OPA548 circuit and load information we could perform a stability analysis and determine what might be done to stabilize it.
Regards, Thomas
Precision Amplifiers Applications Engineering
