AD202/AD204
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 typi-
cal response at the isolator output will be spikes of
±
5 mV
amplitude, 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.
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 ref-
erence 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 output isn’t
needed at all in this application; the whole job is done by the in-
put 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.
6
4
5mΩ
±
20A
M
+7.5V
AD204
200V dc
±
100mV
AD204
±
5V
30k
LOAD
3
38
1
1µF
AD589
R
2
37
V
V
O
=
R
x R
L
R
Figure 20. Motor Control Current Sensing
I
LOAD
=
1.23V
R (2mA MAX)
V
LOAD
≤
4V
Figure 21. Floating Current Source
Photodiode Amplifier.
Figure 22 shows a transresistance con-
nection used to isolate and amplify the output of a photodiode.
The photodiode operates at zero bias, and its output current is
scaled by R
F
to give a +5 V full-scale output.
4
10µA
FS
PHOTO
DIODE
500k
3
AD202
OR
AD204
38
1
0 TO +5V
2
37
Figure 22. Photodiode Amplifier
(Circuit figures shown on this page are for SIP style pack-
ages. Refer to page 3 for proper DIP package pinout.)
REV. B
–11–
AD [ ANALOG DEVICES ]
