Datasheet
AD620
Rev. H | Page 16 of 20
signal according to the following relationship:
)2(2
1
C
D
DIFF
CCR
FilterFreq
+π
=
C
CM
RC
FilterFreq
π
=
2
1
where C
D
≥10C
C.
C
D
affects the difference signal. C
C
affects the common-mode
signal. Any mismatch in R × C
C
degrades the AD620 CMRR. To
avoid inadvertently reducing CMRR-bandwidth performance,
make sure that C
C
is at least one magnitude smaller than C
D
.
The effect of mismatched C
C
s is reduced with a larger C
D
:C
C
ratio.
499Ω
AD620
+
–
V
OUT
R
R
C
C
C
D
C
C
+IN
–IN
REF
–15V
0.1μ F1μ F0
+15V
0.1μ F1μ F0
00775-0-045
Figure 42. Circuit to Attenuate RF Interference
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 43 and Figure 44 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.
REFERENCE
V
OUT
AD620
100
Ω
100
Ω
– INPUT
+ INPUT
AD648
R
G
–V
S
+V
S
–V
S
00775-0-046
Figure 43. Differential Shield Driver
100Ω
– INPUT
+ INPUT
REFERENCE
V
OUT
AD620
–V
S
+V
S
2
R
G
2
R
G
AD548
00775-0-047
Figure 44. 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 45). 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 45.
DIGITAL P.S.
+5VC
ANALOG P.S.
+15V C –15V
AD574A
DIGITAL
DATA
OUTPUT
+
1
μ
F
AD620
0.1
μ
F
AD585
S/H
ADC
0.1
μ
F
1
μ
F
1
μ
F
00775-0-048
Figure 45. Basic Grounding Practice