INA332: What is the maximuim capacitance which can be connected to INA332 output?

Nakai-san,

Even though the Percent Overshoot vs Load Capacitance graph goes out to 10 nF, very high level of overshoot in excess of 75% indicates the Op amp phase margin is 10 degrees, or less. There is a high probability that the output INA332 Op amp is on the verge of becoming unstable and breaking into oscillation. To assure stability we recommend that the phase margin be 45 degrees, or higher. That equates to a to an overshoot of about 25%, or less. Using the graph the maximum capacitance would need to be kept to about 1 nF. That is far less than the 500 nF (0.5 uF) you ask about.

About the only way the INA332 output Op amp would be able to tolerate a 500 nF load would be to add an isolation resistor in series with the output, between the output pin and the 500 nF capacitor. Some calculations would have to be performed to determine the optimum value of this isolation resistor, but before doing that you would need to determine if the circuit that follows the INA332 output can tolerate the series resistance and potential voltage drop that would occur across the resistor.

Additionally, the isolation resistor and 500 nF capacitor would form a low-pass filter. With a capacitor that large the cutoff frequency could be quite low. For example, if the isolation resistor required was 500 Ohms, the -3 dB cutoff frequency would be about 637 Hz. You would need to determine how much bandwidth you need from the INA332 circuit.

Regards, Thomas

Precision Amplifiers Applications Engineering

 Hello Thomas,

 Thank you very much for the prompt reply.

 I could understand well.

 Could I ask you an additional question?

Q. If INA332 output overshoot level on an evaluation board with 0.5uF attaching to the output is less than 25%, 

    can we judge that INA332 can drive the capacitance?

 Thank you very much again and best regards,

 Kazuya. 

Hi Kazuya,

You can certainly try connecting the 0.5 uF capacitor to the INA332 output and then drive its input with a small signal step or square wave. The input amplitude should be kept small, a few millivolts, to keep the Op amps operating small signal and not going into slew rate limiting. Keeping things small signal will allow you to observe the true step response of the amplifier without slew rate entering into the response. You should see a big difference in the overshoot when the INA332 output has no external capacitor and when you connect the 0.5 uF load.

You may find that the INA332 output amplifier doesn't oscillate with the 0.5 uF capacitor connected to the output. Sometimes the capacitor has enough internal equivalent series resistance (ESR) that when added to the load capacitance creates a pole-zero pair that unexpectedly re-compensates the amplifier. The main effect of this besides the amplifier remaining stable is a drastic decrease in the amplifier's bandwidth. Therefore, you wouldn't be able to attain the typical bandwidth performance of the INA332 under this condition.

Do note that this unexpected re-compensation of the amplifier puts your circuit at the mercy of the particular 0.5 uF capacitor type being used. If you change the capacitor type, the ESR may change and the circuit might then become unstable.

Regards, Thomas

Precision Amplifiers Applications Engineering