AD620
+15V
0.1µ F
1 0
µ
F
100
Ω
– INPUT
+V
S
AD648
R
C
C
+IN
499Ω
–IN
+
R
C
D
AD620
–
REF
V
OUT
100
Ω
–V
S
R
G
AD620
V
OUT
C
C
REFERENCE
00775-0-045
0.1µ F
–15V
1 0
µ
F
+ INPUT
–V
S
Figure 43. Circuit to Attenuate RF Interference
Figure 44. Differential Shield Driver
COMMON-MODE REJECTION
Instrumentation amplifiers, such as the AD620, offer high
CMR, which is a measure of the change in output voltage when
both inputs are changed by equal amounts. These specifications
are usually given for a full-range input voltage change and a
specified source imbalance.
For optimal CMR, the reference terminal should be tied to a
low impedance point, and differences in capacitance and
resistance should be kept to a minimum between the two
inputs. In many applications, shielded cables are used to
minimize noise; for best CMR over frequency, the shield
should be properly driven. Figure 44 and Figure 45 show active
data guards that are configured to improve ac common-mode
rejections by “bootstrapping” the capacitances of input cable
shields, thus minimizing the capacitance mismatch between the
inputs.
+V
S
– INPUT
R
G
2
100Ω
AD548
R
G
2
AD620
V
OUT
REFERENCE
+ INPUT
–V
S
Figure 45. Common-Mode Shield Driver
GROUNDING
Since the AD620 output voltage is developed with respect to the
potential on the reference terminal, it can solve many
grounding problems by simply tying the REF pin to the
appropriate “local ground.”
To isolate low level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground pins (Figure 46). It would be
convenient to use a single ground line; however, current
through ground wires and PC runs of the circuit card can cause
hundreds of millivolts of error. Therefore, separate ground
returns should be provided to minimize the current flow from
the sensitive points to the system ground. These ground returns
must be tied together at some point, usually best at the ADC
package shown in Figure 46.
ANALOG P.S.
+15V C –15V
DIGITAL P.S.
C +5V
0.1
µ
F
0.1
µ
F
1
µ
F 1
µ
F
1
µ
F
+
AD620
AD585
S/H
AD574A
ADC
DIGITAL
DATA
OUTPUT
Figure 46. Basic Grounding Practice
Rev. G | Page 17 of 20
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AD [ ANALOG DEVICES ]
