AD202/AD204

REV. B

–11–

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.

AD204

M

± 20A

200V dc

± 5V

± 100mV

5m

Ω

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 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.

3

37

38

AD204

R

4

LOAD

1

2

6

1µF

+7.5V

30k

AD589

R

(2mA MAX)

1.23V

R

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

1

AD202

OR

AD204

38

37

4

3

500k

2

PHOTO

DIODE

0 TO +5V

10µA

FS

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.)