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| 型号： | 05F6950 |
| 英文描述： | IC-COMPUTATION CIRCUIT |
| 中文描述： | 集成电路计算电路 |
| 文件页数： | 11/12页 |
| 文件大小： | 347K |
| 代理商： | 05F6950 |
AD524
REV. C
–8–
INPUT BIAS CURRENTS
Input bias currents are those currents necessary to bias the input
transistors of a dc amplifier. Bias currents are an additional
source of input error and must be considered in an total error
budget. The bias currents when multiplied by the source resis-
tance appear as an offset voltage. What is of concern in calculat-
ing bias current errors is the change in bias current with respect
to signal voltage and temperature. Input offset current is the dif-
ference between the two input bias currents. The effect of offset
current is an input offset voltage whose magnitude is the offset
current times the source impedance imbalance.
Although instrumentation amplifiers have differential inputs,
there must be a return path for the bias currents. If this is not
provided, those currents will charge stray capacitances, causing
the output to drift uncontrollably or to saturate. Therefore,
when amplifying “floating” input sources such as transformers
and thermocouples, as well as ac-coupled sources, there must
still be a dc path from each input to ground.
7
–Vs
8
+Vs
AD524
6
9
3
LOAD
TO POWER
SUPPLY
GROUND
2
11
12
13
16
1
10
a. Transformer Coupled
AD524
6
9
3
LOAD
TO POWER
SUPPLY
GROUND
2
11
12
13
16
1
10
7
–Vs
8
+Vs
b. Thermocouple
AD524
6
9
3
LOAD
TO POWER
SUPPLY
GROUND
2
11
12
13
16
1
10
7
–Vs
8
+Vs
c. AC Coupled
Figure 34. Indirect Ground Returns for Bias Currents
Table I. Output Gain Resistor Values
Output
Nominal
Gain
R2
R1, R3
Gain
2
5 k
2.26 k
2.02
5
1.05 k
2.05 k
5.01
10
1 k
4.42 k
10.1
COMMON-MODE REJECTION
Common-mode rejection is a measure of the change in output
voltage when both inputs are changed equal amounts. These
specifications are usually given for a full-range input voltage
change and a specified source imbalance. “Common-Mode Re-
jection Ratio” (CMRR) is a ratio expression while “Common-
Mode Rejection” (CMR) is the logarithm of that ratio. For
example, a CMRR of 10,000 corresponds to a CMR of 80 dB.
In an instrumentation amplifier, ac common-mode rejection is
only as good as the differential phase shift. Degradation of ac
common-mode rejection is caused by unequal drops across dif-
fering track resistances and a differential phase shift due to var-
ied stray capacitances or cable capacitances. In many
applications shielded cables are used to minimize noise. This
technique can create common mode rejection errors unless the
shield is properly driven. Figures 35 and 36 shows active data
guards which are configured to improve ac common mode rejec-
tion by “bootstrapping” the capacitances of the input cabling,
thus minimizing differential phase shift.
1
12
3
2
10
6
9
8
+Vs
7
–Vs
100
AD711
VOUT
REFERENCE
AD524
G=100
RG2
+INPUT
–INPUT
Figure 35. Shield Driver, G
≥ 100
–Vs
100
REFERENCE
1
12
3
2
10
8
+Vs
7
–Vs
VOUT
AD524
16
100
6
AD712
–INPUT
RG1
+INPUT
RG2
9
Figure 36. Differential Shield Driver
GROUNDING
Many data-acquisition components have two or more ground
pins which are not connected together within the device. These
grounds must be tied together at one point, usually at the sys-
tem power-supply ground. Ideally, a single solid ground would
be desirable. However, since current flows through the ground
7
9
11
15
1
DIGITAL P.S.
+5V
C
–15V
ANALOG P.S.
0.1
F
1F 1F
1F
0.1
F
DIG
COM
AD574A
C
+15V
8
7
10
9
6
0.1
F
0.1
F
OUTPUT
REFERENCE
ANALOG
GROUND
*
AD524
DIGITAL
DATA
OUTPUT
SIGNAL
GROUND
IF INDEPENDENT, OTHERWISE
RETURN AMPLIFIER REFERENCE
TO MECCA AT ANALOG P.S. COMMON
*
2
1
AD583
SAMPLE
AND HOLD
Figure 37. Basic Grounding Practice
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