OP1177_07 Datasheet, PDF (17/24 Pages) Analog Devices – Precision Low Noise, Low Input Bias Current Operational Amplifiers

OP1177/OP2177/OP4177
+
400mV _–
V+
2
7
6
OP1177
3
4
V–
V_OUT
R_S
C_L
C_S
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
1+
R2
R1
(1+ sC_t R1)
Cf
R1
R2
+
V1
–
Ct
V+
2
7
OP1177 ⁶
3
4
V_OUT
V–
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.
R1
R2
indicating a zero at
s
=
R2 + R1
R2R1C_t
=
1
2π (R1/R2)C_t
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.
+
V1
–
V+
2
7
C
OP1177 ⁶
3
4
V–
Figure 61. EMI Reduction
V_OUT
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.
Setting Cf = (R1/R2) Ct achieves a phase margin of 90°.
R1
R2
+
V1
–
V+
2
7
6
Ct
OP1177
3
4
V_OUT
V–
Figure 59. Stray Input Capacitance
R can be chosen independently of C for a specific phase margin
according to the formula
R
=
R2
a ( jf ₂ )
−
⎜⎝⎛¹
+
R2
R1
⎟⎠⎞
where:
a is the open-loop gain of the amplifier.
f2 is the frequency at which the phase of a = ΦM − 180°.
R2
R1
+
R
V1
–
C
V+
2
7
OP1177 ⁶
3
4
V_OUT
V–
Figure 62. Compensation Using Input R-C Network
Rev. E | Page 17 of 24