REV. D

AD202/AD204

–4–

DIFFERENCES BETWEEN THE AD202 AND AD204

The primary distinction between the AD202 and AD204 is in

the method by which they are powered: the AD202 operates

directly from 15 V dc while the AD204 is powered by a non-

isolated externally-supplied clock (AD246) that can drive up to

32 AD204s. The main advantages of using the externally-

clocked AD204 over the AD202 are reduced cost in multichannel

applications, lower power consumption, and higher bandwidth.

In addition, the AD204 can supply substantially more isolated

power than the AD202.

Of course, in a great many situations, especially where only one

or a few isolators are used, the convenience of standalone opera-

tion provided by the AD202 will be more significant than any

of the AD204’s advantages. There may also be cases where it is

desirable to accommodate either device interchangeably, so the

pinouts of the two products have been designed to make that

easy to do.

RECT

AND

FILTER

OSCILLATOR

DEMOD

MOD

SIGNAL

POWER

5V

FS

+7.5V

–7.5V

25kHz

25kHz

AD202

FB

IN–

IN+

IN COM

5V

FS

HI

LO

15V DC

POWER

RETURN

Figure 1a. AD202 Functional Block Diagram

RECT

AND

FILTER

POWER

CONV.

DEMOD

MOD

SIGNAL

POWER

5V

FS

+7.5V

–7.5V

25kHz

25kHz

AD204

FB

IN–

IN+

IN COM

5V

FS

HI

LO

CLOCK

15V p-p

25kHz

POWER

RETURN

Figure 1b. AD204 Functional Block Diagram

(Pin Designations Apply to the DIP-Style Package)

INSIDE THE AD202 AND AD204

The AD202 and AD204 use an amplitude modulation technique

to permit transformer coupling of signals down to dc (Figure 1a

and 1b). Both models also contain an uncommitted input op

amp and a power transformer that provides isolated power to

the op amp, the modulator, and any external load. The power

transformer primary is driven by a 25 kHz, 15 V p-p square

wave generated internally in the case of the AD202, or supplied

externally for the AD204.

Within the signal swing limits of approximately

±5 V, the out-

put voltage of the isolator is equal to the output voltage of the

op amp; that is, the isolation barrier has unity gain. The output

signal is not internally buffered, so the user is free to interchange

the output leads to get signal inversion. Additionally, in multi-

channel applications, the unbuffered outputs can be multiplexed

with one buffer following the mux. This technique minimizes

offset errors while reducing power consumption and cost. The

output resistance of the isolator is typically 3 k

Ω for the AD204

(7 k

Ω for AD202) and varies with signal level and temperature,

so it should not be loaded (see Figure 2 for the effects of load

upon nonlinearity and gain drift). In many cases, a high imped-

ance load will be present or a following circuit such as an output

filter can serve as a buffer so that a separate buffer function will

not often be needed.

OUTPUT LOAD – M

0.25

0.20

0

0

1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.15

0.10

0.05

–10

–8

–6

–4

–2

0

–500

–400

–300

–200

–100

0

GAIN

CHANGE

(%)

GAIN TC

CHANGE

(ppm/ C)

NON-

LINEARITY

(%)

AD202 GAIN AND GAIN TC

AD202 NONLINEARITY

AD204 NONLINEARITY

AD204 GAIN AND GAIN TC

Figure 2. Effects of Output Loading

USING THE AD202 AND AD204

Powering the AD202. The AD202 requires only a single 15 V

power supply connected as shown in Figure 3a. A bypass capaci-

tor is provided in the module.

15V

5%

15V RETURN

AD202

Figure 3a.

Powering the AD204. The AD204 gets its power from an

externally supplied clock signal (a 15 V p-p square wave with a

nominal frequency of 25 kHz) as shown in Figure 3b.

15V

15V RETURN

AD204

AD204

AD204

AD246

+

Figure 3b.

(NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to

Page 3 for proper DIP package pinout.)