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| 型號： | AN-242 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | LJT 18C 18#20 SKT RECP |
| 中文描述： | 應用新的精密運算放大器 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大?。?/td> | 267K |
| 代理商： | AN-242 |
The simplest connection for making a high-input-impedance
differential amplifier using op amps is shown inFigure 20. Its
main disadvantage is that the common-mode signal on the
inverting input is delayed by the response of A1 before be-
ing delivered to A2 for cancellation. A selected capacitor
across R1 will compensate for this, but AC common-mode
rejection will deteriorate as the characteristics of A1 vary
with temperature.
f
2
j
10 Hz
*
gain set
2
trim for DC CMRR
TL/H/7479–22
Figure 20. Two-op-amp instrumentation amplifier has
poor AC common-mode rejection. This can
be improved at the expense of differential
bandwidth with C2.
When slowly varying differential signals are of interest, the
response of A2 can be rolled off with C2 to reduce the sen-
sitivity of the circuit to high frequency common-mode sig-
nals. If single-resistor gain setting is desired, R5 can be add-
ed. Otherwise, it is unnecessary.
A full-blown differential amplifier with extremely high input
impedance is shown inFigure 21. Gain is fixed at 1000, but
it can be varied with R10. Differential offset balancing is
provided on both input amplifiers by R18.
The AC common-mode rejection is dependent on how well
the frequency characteristics of A2 and A3 match. This is a
far better situation than encountered with the previous cir-
cuit. When AC rejection must be optimized, amplifier differ-
ences as well as the effects of unbalanced stray capaci-
tances can be compensated for with a capacitor across R13
or R14, depending on which side is slower. Alternately, C1
can be added to control the differential bandwidth and make
AC common-mode rejection less dependent on amplifier
matching. The value shown gives approximately 100 Hz dif-
ferential bandwidth, although it will vary with gain setting.
A separate amplifier is used to drive the shields of the input
cables. This reduces cable leakage currents and spurious
signals generated from cable flexing. It may also be required
to neutralize cable capacitance. Even short cables can at-
tenuate low-frequency signals with high enough source re-
sistance. Another balance potentiometer, R8, is included so
that resistor mismatches in the drive to the bootstrapping
amplifier can be neutralized. Adding the bootstrapping am-
plifier also provides a connection point, as shown, for bias-
current compensation if the ultimate in performance is re-
quired.
22
current zero
3
voltage balance
*
gain
2
DC CMRR
**
AC CMRR
TL/H/7479–23
Figure 21. High gain differential instrumentation amplifier includes input guarding, cable bootstrapping and bias cur-
rent compensation. Differential bandwidth is reduced by C1 which also makes common-mode rejection less
dependent on matching of input amplifiers.
12
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