AD202/AD204
CHANNEL 0
HI
LO
CHANNEL 1
HI
LO
Figure 17. A three-pole active filter is included in the design to
get normal-mode rejection of frequencies above a few Hz and to
provide enhanced common-mode rejection at 60 Hz. If offset
adjustment is needed, it is best done at the trim pins of the OP07
itself; gain adjustment can be done at the feedback resistor.
Note that the isolated supply current is large enough to mandate
the use of 1
mF
supply bypass capacitors. This circuit can be
used with an AD202 if a low power op amp is used instead of
the OP07.
Process Current Input with Offset.
Figure 18 shows an
isolator receiver that translates a 4-20 mA process current
signal into a 0 V to 10 V output. A 1 V to 5 V signal appears at
the isolator’s output, and a –1 V reference applied to output LO
provides the necessary level shift (in multichannel applications,
the reference can be shared by all channels). This technique is
often useful for getting offset with a follower-type output buffer.
AD202
OR
AD204
+
4–20mA
250
–
–15V
R
F
0.1”
GRID
R
G
R
F
R
G
100pF
100pF
1
2
3
4
5
6
1
2
3
4
5
6
+15V
1V
TO
5V
15k
+
Figure 16b.
0V
TO
10V
–
Synchronization.
Since AD204s operate from a common
clock, synchronization is inherent. AD202s will normally not
interact to produce beat frequencies even when mounted on
0.25-inch centers. Interaction may occur in rare situations where
a large number of long, unshielded input cables are bundled
together and channel gains are high. In such cases, shielded
cable may be required or AD204s can be used.
APPLICATIONS EXAMPLES
–1V TO
ADDITIONAL
CHANNELS
1k
237
10k
AD589
6.8k
–15V
Figure 18. Process Current Input Isolator with Offset
Low Level Sensor Inputs.
In applications where the output of
low level sensors such as thermocouples must be isolated, a low
drift input amplifier can be used with an AD204, as shown in
The circuit as shown requires a source compliance of at least
5 V, but if necessary that can be reduced by using a lower value
of current-sampling resistor and configuring the input amplifier
for a small gain.
0.15 F
AD204
HI
39k
+
1 F
AD OP-07
49.9k
470k
470k
0.038 F
+
V
O
= V
I
–
(
1 + 50k
)
R
G
R
G
LO
220M
OPTIONAL
OPEN INPUT
DETECTION
1 F
+
1 F
CLK
+7.5V
–7.5V
CLK RET
+
Figure 17. Input Amplifier and Filter for Sensor Signals
(NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to
Page 3 for proper DIP package pinout.)
REV. D
–9–
AD [ ANALOG DEVICES ]
