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7.3.5 Driving Capacitive Loads

The OPA838 can drive small capacitive loads directly without oscillation (less than 6@pF). When driving

capacitive loads greater than 6 pF, Figure 6-49 illustrates the recommended ROUT vs capacitor load parametric

on gains. At higher gains, the amplifier starts with greater phase margin into a resistive load and can operate

with lower ROUT for a given capacitive load. Without ROUT, output capacitance interacts with the output

impedance of the amplifier, which causes phase shift in the loop gain of the amplifier that reduces the phase

margin. This causes peaking in the frequency response with overshoot and ringing in the pulse response.

Figure 6-49 targets a 30° phase margin for the OPA838. A 30° phase margin produces a 5.7

‑dB peaking in

the frequency response at the amplifier output pin that is rolled off by the output RC pole; see Figure 7-7. This

peaking can cause clipping for large signals driving a capacitive load. Increasing the ROUT value can reduce the

peaking at the cost of a more band-limited overall response.

F 1

NŸ

G 200

OUT

V1 2.5 V

V2 ±2.5 V

VG1

OUT 68.1

Figure 7-6. ROUT versus CL Test Circuit

Frequency (Hz)

-5

-2

10k

100k

10M

100M

CapL

At capacitive load

At output pin

Figure 7-7. Frequency Response to Output Pin and Capacitive Load

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OPA838

SBOS867D – AUGUST 2017 – REVISED SEPTEMBER 2024

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