10

Rev. B.01

www.3peakic.com.cn

LM321 / LM358 / LM324

1.2MHz, Low-Power 36V Op Amps

10k

Ω

5pF

10k

Ω

Figure 2

Driving Large Capacitive Load

The LM321/358/324 of OPA is designed to drive large capacitive loads. Refer to Typical Performance Characteristics

for “Phase Margin vs. Load Capacitance”. As always, larger load capacitance decreases overall phase margin in a

feedback system where internal frequency compensation is utilized.

As the load capacitance increases, the feedback

loop’s phase margin decreases, and the closed-loop bandwidth is reduced. This produces gain peaking in the

frequency response, with overshoot and ringing in output step response. The unity-gain buffer (G = +1V/V) is the most

sensitive to large capacitive loads.

When driving large capacitive loads with the LM321/358/324 (e.g., > 200 pF when G = +1V/V), a small series resistor

resistive at higher frequencies.

Figure 3

Power Supply Layout and Bypass

The LM321/358/324 OPA

’s power supply pin should have a local bypass capacitor (i.e., 0.01μF to 0.1μF) within 2mm

for good high frequency performance. It can also use a bulk capacitor

(i.e., 1μF or larger) within 100mm to provide

large, slow currents. This bulk capacitor can be shared with other analog parts.

Ground layout improves performance by decreasing the amount of stray capacitance and noise at the OPA

’s inputs

and outputs. To decrease stray capacitance, minimize PC board lengths and resistor leads, and place external

components

as close to the op amps’ pins as possible.

Proper Board Layout

To ensure optimum performance at the PCB level, care must be taken in the design of the board layout. To avoid

leakage currents, the surface of the board should be kept clean and free of moisture. Coating the surface creates a

barrier to moisture accumulation and helps reduce parasitic resistance on the board.

Keeping supply traces short and properly bypassing the power supplies minimizes power supply disturbances due to

output current variation, such as when driving an ac signal into a heavy load. Bypass capacitors should be connected

as closely as possible to the device supply pins. Stray capacitances are a concern at the outputs and the inputs of the

amplifier. It is recommended that signal traces be kept at least 5mm from supply lines to minimize coupling.

A variation in temperature across the PCB can cause a mismatch in the Seebeck voltages at solder joints and other

points where dissimilar metals are in contact, resulting in thermal voltage errors. To minimize these thermocouple

effects, orient resistors so heat sources warm both ends equally. Input signal paths should contain matching numbers