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OP4177ARZ1 参数 Datasheet PDF下载

OP4177ARZ1图片预览
型号: OP4177ARZ1
PDF下载: 下载PDF文件 查看货源
内容描述: 精密,低噪声,低输入偏置电流运算放大器 [Precision Low Noise, Low Input Bias Current Operational Amplifiers]
分类和应用: 运算放大器
文件页数/大小: 24 页 / 479 K
品牌: ADI [ ADI ]
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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