AD202/AD204
Synchronization.
Since AD204s operate from a common
clock, synchronization is inherent. AD202s will normally not in-
teract 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 to-
gether and channel gains are high. In such cases, shielded cable
may be required or AD204s can be used.
APPLICATIONS EXAMPLES
Process Current Input with Offset.
Figure 18 shows an iso-
lator receiver which 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.
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 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.
4
4
AD202
OR
AD204
38
+15V
3
7
6
1V TO 5V 2
4
3
1
4-20mA
250Ω
2
37
15k
0 TO +10V
10k
–15V
–1V TO
ADDITIONAL
CHANNELS
237Ω
1k
AD589
AD OP-07
39k
HI
2
4
7
3
6
0.15µF
3
6.8k
38
470k
49.9k
470k
1
–15V
0.039µF
2
V
O
= V
I
x 1 + 50k
R
G
37
(
)
Figure 18. Process Current Input Isolator with Offset
1µF
LO
R
G
AD204
1µF
220M
6
1µF
33
CLK
+7.5V
32
OPTIONAL
OPEN INPUT
DETECTION
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.
High-Compliance Current Source.
In Figure 19, an isolator
is used to sense the voltage across current-sensing resistor R to
allow direct feedback control of a high-voltage transistor or FET
used as a high-compliance current source. Since the isolator has
virtually no response to dc common-mode voltage, the closed-
loop current source has a static output resistance greater than
10
14
Ω
even for output currents of several mA. The output cur-
rent capability of the circuit is limited only by power dissipation
in the source transistor.
–10V TO +250V
~
~
5
–7.5V
CLK RET
Figure 17. Input Amplifier & Filter for Sensor Signals
Note that the isolated supply current is large enough to mandate
the use of 1
µF
supply bypass capacitors. This circuit can be
used with an AD202 if a low-power op amp is used instead of
the OP07.
~
4
I
L
= V
C
R
S
LOAD
3
1
AD202
OR
AD204
38
R
S
1kΩ
2
37
470pF
+15V
100k
~
+5V REF
20k
MPS
U10
1k
10k
V
C
–15V
~
Figure 19. High-Compliance Current Source
(Circuit figures shown on this page are for SIP style pack-
ages. Refer to page 3 for proper DIP package pinout.)
–10–
REV. B
AD [ ANALOG DEVICES ]
