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) 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.
FB
IN–
IN+
V
SIG
IN COM
+V
ISO
OUT
–V
ISO
OUT
+7.5V
–7.5V
RECT
AND
FILTER
POWER
OSCILLATOR
25kHz
25kHz
15V DC
5V
FS
MOD
5V
FS
SIGNAL
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 kΩ for the AD204
(7 kΩ 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.
NON-
LINEARITY
(%)
0.25
GAIN
GAIN TC
CHANGE CHANGE
(%)
(ppm/ C)
–10
–500
0.20
–8
–400
AD202
DEMOD
HI
V
OUT
LO
0.15
AD202 GAIN AND GAIN TC
AD202 NONLINEARITY
–6
–300
0.10
AD204 GAIN AND GAIN TC
–4
–200
0.05
–2
–100
AD204 NONLINEARITY
POWER
RETURN
0
0
0.1
0.2
0.3
0.4
0.5
0.6
OUTPUT LOAD – M
0.7
0.8
0.9
0
1.0
0
Figure 1a. AD202 Functional Block Diagram
FB
IN–
IN+
V
SIG
IN COM
+V
ISO
OUT
–V
ISO
OUT
+7.5V
–7.5V
RECT
AND
FILTER
POWER
25kHz
POWER
CONV.
CLOCK
15V p-p
25kHz
POWER
RETURN
5V
FS
MOD
5V
FS
SIGNAL
Figure 2. Effects of Output Loading
USING THE AD202 AND AD204
AD204
DEMOD
HI
LO
V
OUT
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.
AD202
15V
5%
25kHz
15V RETURN
Figure 1b. AD204 Functional Block Diagram
(Pin Designations Apply to the DIP-Style Package)
Figure 3a.
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
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.
AD246
AD204
AD204
AD204
+
15V
15V RETURN
Figure 3b.
(NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to
Page 3 for proper DIP package pinout.)
–4–
REV. D
AD [ ANALOG DEVICES ]
