SGM8651/2/3/4/5

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APPLICATION NOTES

Driving Capacitive Loads

The SGM865x can directly drive 47pF in unity-gain without

oscillation. The unity-gain follower (buffer) is the most sensitive

configuration to capacitive loading. Direct capacitive loading

reduces the phase margin of amplifiers and this results in

ringing or even oscillation. Applications that require greater

capacitive drive capability should use an isolation resistor

between the output and the capacitive load like the circuit in

SGM8651

Figure 1. Indirectly Driving Heavy Capacitive Load

An improvement circuit is shown in Figure 2. It provides DC

serve to counteract the loss of phase margin by feeding the

high frequency component of the output signal back to the

amplifier’s inverting input, thereby preserving phase margin in

the overall feedback loop.

SGM8651

Figure 2. Indirectly Driving Heavy Capacitive Load with DC

Accuracy

For no-buffer configuration, there are two others ways to

increase the phase margin: (a) by increasing the amplifier’s

gain or (b) by placing a capacitor in parallel with the feedback

resistor to counteract the parasitic capacitance associated with

inverting node.

Power-Supply Bypassing and Layout

The SGM865x family operates from either a single +2.7V to

+5.5V supply or dual ±1.35V to ±2.75V supplies. For

0.1µF ceramic capacitor which should be placed close to the

supplies should be bypassed to ground with separate 0.1µF

ceramic capacitors. 2.2µF tantalum capacitor can be added for

better performance.

Good PC board layout techniques optimize performance by

decreasing the amount of stray capacitance at the op amp’s

inputs and output. To decrease stray capacitance, minimize

trace lengths and widths by placing external components as

close to the device as possible. Use surface-mount

components whenever possible.

For the high speed operational amplifier, soldering the part to

the board directly is strongly recommended. Try to keep the

high frequency big current loop area small to minimize the EMI

(electromagnetic interfacing).

SGM8651

Vn

Vp

10µF

0.1µF

10µF

0.1µF

SGM8651

Vn

Vp

10µF

0.1µF

Figure 3. Amplifier with Bypass Capacitors

Grounding

A ground plane layer is important for high speed circuit design.

The length of the current path speed currents in an inductive

ground return will create an unwanted voltage noise. Broad

ground plane areas will reduce the parasitic inductance.

Input-to-Output Coupling

To minimize capacitive coupling, the input and output signal

traces should not be parallel. This helps reduce unwanted

positive feedback.