OP1177/OP2177/OP4177
C
f
V+
7
2
3
R1
R2
V+
6
V
OUT
OP1177
R
+
–
S
400mV
C
L
4
+
–
C
S
7
2
3
V1
V–
6
V
OUT
C
t
OP1177
Figure 58. Snubber Network Configuration
4
Caution: The snubber technique cannot recover the loss of
bandwidth induced by large capacitive loads.
V–
Figure 60. Compensation Using Feedback Capacitor
STRAY INPUT CAPACITANCE COMPENSATION
REDUCING ELECTROMAGNETIC INTERFERENCE
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.
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.
The transfer function of the circuit is
R2
R1
1+
(
1+ sCt R1
)
R1
R2
V+
indicating a zero at
+
–
7
2
3
R2 + R1
s =
1
V1
=
6
V
OUT
OP1177
R2R1Ct
2π
(
R1/R2
)
Ct
C
4
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.
V–
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.
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°.
R can be chosen independently of C for a specific phase margin
according to the formula
R1
R2
R2
jf2
R2
R1
⎛
⎝
⎞
⎟
⎠
V+
R =
− 1 +
⎜
+
–
a
(
)
7
2
3
V1
6
V
where:
OUT
C
t
OP1177
a is the open-loop gain of the amplifier.
4
f2 is the frequency at which the phase of a = ΦM − 180°.
V–
R2
Figure 59. Stray Input Capacitance
V+
R1
7
2
+
–
R
6
V
OUT
OP1177
V1
C
3
4
V–
Figure 62. Compensation Using Input R-C Network
Rev. G | Page 17 of 24
ADI [ ADI ]
