REV. B

–4–

AD202/AD204

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) which 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 stand-alone op-

eration 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.

MOD

RECT &

FILTER

3

2

1

4

SIGNAL

DEMOD

±5V

F.S.

25kHz

25kHz

POWER

OSCILLATOR

FB

IN–

IN+

IN COM

38

37

31

RETURN

+15V DC

HI

LO

AD202

+7.5V

–7.5V

±5V

FS

6

5

Figure 1a. AD202 Functional Block Diagram

MOD

RECT &

FILTER

3

2

1

4

SIGNAL

±5V

F.S.

25kHz

25kHz

POWER

FB

IN–

IN+

IN COM

33

RETURN

(15V

p-p/25kHz)

HI

LO

AD204

+7.5V

–7.5V

±5V

FS

6

5

CLOCK

POWER

CONV.

DEMOD

38

37

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 which 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 which is generated internally in the case of the AD202, or

supplied externally for the AD204.

Within the signal swing limits of approximately

±5 V, the output

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

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

nal 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

(Circuit figures shown on this page are for SIP style pack-

ages. Refer to page 3 for proper DIP package pinout.)

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.

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 ca-

pacitor is provided in the module.

Figure 3a.

Powering the AD204.

The AD204 gets its power from an ex-

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

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

Figure 3b.

AD246 Clock Driver.

The AD246 is a compact, inexpensive

clock driver that can be used to obtain the required clock from a

single 15 V supply. Alternatively, the circuit shown in Figure 4

(essentially an AD246) can be used. In either case, one clock

circuit can operate at least 32 AD204s at the rated minimum

supply voltage of 14.25 V and one additional isolator can be

operated for each 40 mV increase in supply voltage up to 15 V.

A supply bypass capacitor is included in the AD246, but if many