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OPA2322: capacitive load capability at unity gain

Takao Yamamoto
Genius 4135 points
Other Parts Discussed in Thread: OPA322

Dear all,

Could you teach me about guaranteed value of capacitive load at unity gain?

At data sheet, you mentioned that this device can 1000pF load at unity gain.

Best regards,

  • 0
    Genius 4135 points
    Dear all,

    I would appreciate if you answer as soon as possible.

    Best regards,
  • 0 in reply to Takao Yamamoto
    TI__Genius 15410 points

    Hello,

    I believe you are refering to the paragraph on page 14 of the datasheet discussing CAPACITIVE LOAD AND STABILITY. The paragraph states that the OPA322 will remain stable in unity-gain with a pure capactive load of 1nF. Figure 33 shows a graph for % Overshoot vs. Capacitive Load, at 1nF (1000pF) the graph shows that the OPA322 will have approximately 63% overshoot. This translates to about 15 degree of phase margin. While 15 degrees of phase margin is technically stable because it is greater than 0 degrees of phase margin, it is recommended to always design for at least 45 degrees of phase margin. Therefore, I would recommend only driving a purely capactive load of ~200pF to keep the phase margin of the OPA322 above 45 degrees (~23% Overshoot).

    If a capacitive load larger than 200pF is required a small series resistor can be inserted between the op amp output and the capacitive load as stated in the second paragraph of the the CAPACITIVE LOAD AND STABILITY section and shown in Figure 34. Please see our TI Design, TIPD128, that discusses how to stabilize an op amp with a capacitive load using a series output resistor.

    For more information on stability, I recommend checking out our TI Precision Lab videos on Op Amp Stability.

    -Tim Claycomb