OP4177 Analog Devices, OP4177 Datasheet - Page 17

• Manufacturer Part Number: OP4177
• Description: Precision Low Noise, Low Input Bias Current Quad Op Amp
• Manufacturer: Analog Devices

Specifications of OP4177

• -3db Bandwidth: 1.3MHz
• Slew Rate: 700mV/µs
• Vos: 15µV
• Ib: 500pA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 7.9nV/rtHz
• Vcc-vee: 5V to 36V
• Isy Per Amplifier: 500µA
• Packages: SOIC,SOP

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 (C
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
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 (Φ
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 C
1
s
+
=
400mV
R2
R1
+
–
f
t
) consists of three components. These are the internal
R2R1C
R2
= (R1/R2) C
(
V1
1
+
Figure 58. Snubber Network Configuration
+
–
+
R1
sC
R1
t
Figure 59. Stray Input Capacitance
t
=
C t
R1
2
3
2
OP1177
)
π
t
achieves a phase margin of 90°.
(
7
4
R1
V–
V+
2
3
1
/
R2
OP1177
R2
)
7
6
M
4
V–
V+
C
) can be severely degraded,
t
6
R
C
S
S
C
V
L
OUT
V
OUT
Rev. G | Page 17 of 24
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.
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.
f
2
is the frequency at which the phase of a = Φ
R
=
+
–
+
–
+
–
Figure 60. Compensation Using Feedback Capacitor
a
Figure 62. Compensation Using Input R-C Network
(
R2
V1
V1
V1
jf
R1
2
R1
R1
)
C
R
−
C t
C
⎛ +
⎜
⎝
Figure 61. EMI Reduction
1
OP1177/OP2177/OP4177
R2
R1
2
3
2
3
2
3
OP1177
OP1177
OP1177
⎞
⎟
⎠
R2
C f
R2
R2
7
4
7
V+
V–
4
7
4
V+
V–
V–
V+
6
6
6
M
− 180°.
V
V
V
OUT
OUT
OUT