Datasheet
REV. C
AD202/AD204
–10–
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
current capability of the circuit is limited only by power dissipa-
tion in the source transistor.
AD202
OR
AD204
100k
+15V
–15V
470pF
10k
+5V REF
+
–
V
C
20k
LOAD
I
L
=
V
C
R
S
–10V TO +250V
R
S
1k
MPS
U10
1k
Figure 19. High-Compliance Current Source
Motor Control Isolator. The AD202 and AD204 perform
very well in applications where rejection of fast common-mode
steps is important but bandwidth must not be compromised.
Current sensing in a fill-wave bridge motor driver (Figure 20) is
one example of this class of application. For 200 V common-mode
steps (1 µs rise time) and a gain of 50 as shown, the typical
response at the isolator output will be spikes of ±5 mV ampli-
tude, decaying to zero in less than 100 µs. Spike height can be
reduced by a factor of four with output filtering just beyond the
isolator’s bandwidth.
AD204
+
–
5V
M
5m
20A
+
200V dc
–
100mV
Figure 20. Motor Control Current Sensing
Floating Current Source/Ohmmeter. When a small floating
current is needed with a compliance range of up to ±1000 V dc,
the AD204 can be used to both create and regulate the current.
This can save considerable power, since the controlled current
does not have to return to ground. In Figure 21, an AD589
reference is used to force a small fixed voltage across R. That
sets the current which the input op amp will have to return
through the load to zero its input. Note that the isolator’s out-
put isn’t needed at all in this application; the whole job is done
by the input section. However, the signal at the output could be
useful it’s the voltage across the load, referenced to ground. Since
the load current is known, the output voltage is proportional to
load resistance.
AD204
LOAD
R
7.5V
1F
+
30k
AD589
+
–
V
O
= R
L
V
R
R
I
LOAD
= (2mA MAX)
V
LOAD
4V
1.23V
R
Figure 21. Floating Current Source
Photodiode Amplifier. Figure 22 shows a transresistance
connection used to isolate and amplify the output of a photo-
diode. The photodiode operates at zero bias, and its output
current is scaled by R
F
to give a 5 V full-scale output.
AD202
OR
AD204
0V TO 5V
500k
PHOTO
DIODE
10A
FS
Figure 22. Photodiode Amplifier
(NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to
Page 3 for proper DIP package pinout.)