AD204(1994) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD204 Datasheet PDF : 12 Pages
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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.
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.
APPLICATIONS EXAMPLES
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
AD OP-07
39k 3
0.15µF
3
HI
6 470k 470k
1
2
7
4
49.9k
0.039µF
2
1µF
RG
LO
220M
OPTIONAL
OPEN INPUT
DETECTION
1µF
1µF
AD204
6 +7.5V
5 –7.5V
38
( ) VO = VI x
1 + 50k
RG
37
33 CLK
32 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
4-20mA
3
1
250Ω
2
AD202
OR
AD204
+15V
3
38
1V TO 5V 2
37
7
6
4 15k
–15V
–1V TO
1k
ADDITIONAL
10k
CHANNELS
237Ω AD589
0 TO +10V
6.8k
–15V
Figure 18. Process Current Input Isolator with Offset
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
1014 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
4
IL = VC
LOAD
RS
3
1
RS
1kΩ
2
AD202
OR
AD204
38
37
MPS
U10
10k
1k
470pF
~+15V
~
–15V
100k
+5V REF
20k
VC
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
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