REV. D

AD202/AD204

–10–

High Compliance Current Source. In Figure 19, an isolator

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

W 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

+

–

20k

LOAD

–10V TO +250V

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

ms 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

ms. 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 that 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 as it’s the voltage across the load, referenced to ground.

Since the load current is known, the output voltage is propor-

tional to load resistance.

AD204

LOAD

R

7.5V

1 F

+

30k

AD589

+

–

R

(2mA MAX)

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

AD202

OR

AD204

0V TO 5V

500k

PHOTO

DIODE

10 A

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.)