OP1177ARMZ-R2 datasheet(17/24 Pages) AD

17 / 24 page

OP1177/OP2177/OP4177

Rev. E | Page 17 of 24

OP1177

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VOUT

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400mV

Figure 58. Snubber Network Configuration

Caution: The snubber technique cannot recover the loss of

bandwidth induced by large capacitive loads.

STRAY INPUT CAPACITANCE COMPENSATION

The effective input capacitance in an operational amplifier

circuit (Ct) consists of three components. These are the internal

differential capacitance between the input terminals, the internal

common-mode capacitance of each input to ground, and the

external capacitance including parasitic capacitance. In the

circuit in Figure 59, the closed-loop gain increases as the signal

frequency increases.

The transfer function of the circuit is

()

indicating a zero at

()

R2R1C

Depending on the value of R1 and R2, the cutoff frequency of

the closed-loop gain can be well below the crossover frequency.

In this case, the phase margin (ΦM) can be severely degraded,

resulting in excessive ringing or even oscillation.

A simple way to overcome this problem is to insert a capacitor

in the feedback path, as shown in Figure 60.

The resulting pole can be positioned to adjust the phase margin.

Setting Cf = (R1/R2) Ct achieves a phase margin of 90°.

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Figure 59. Stray Input Capacitance

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Figure 60. Compensation Using Feedback Capacitor

REDUCING ELECTROMAGNETIC INTERFERENCE

A number of methods can be utilized to reduce the effects of

EMI on amplifier circuits.

In one method, stray signals on either input are coupled to the

opposite input of the amplifier. The result is that the signal is

rejected according to the CMRR of the amplifier.

This is usually achieved by inserting a capacitor between the inputs

of the amplifier, as shown in Figure 61. However, this method can

also cause instability, depending on the value of capacitance.

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Figure 61. EMI Reduction

Placing a resistor in series with the capacitor (see Figure 62)

increases the dc loop gain and reduces the output error. Positioning

the breakpoint (introduced by R-C) below the secondary pole of

the operational amplifier improves the phase margin and,

therefore, stability.

R can be chosen independently of C for a specific phase margin

according to the formula

()

⎟

⎠

⎞

⎜

⎝

⎛ +

−

where:

a is the open-loop gain of the amplifier.

f2 is the frequency at which the phase of a = ΦM − 180°.

OP1177

VOUT

–

V+

V–

Figure 62. Compensation Using Input R-C Network

OP1177ARMZ-R2 Datasheet(PDF) 17 Page - Analog Devices

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