AD202/AD204
Using Isolated Power.
Both the AD202 and the AD204 provide
±7.5
V power outputs referenced to input common. These may be
used to power various accessory circuits that must operate at
the input common-mode level; the input zero adjustment pots
described above are an example, and several other possible uses
are shown in the section titled Application Examples.
The isolated power output of the AD202 (400
mA
total from
either or both outputs) is much more limited in current capacity
than that of the AD204, but it is sufficient for operating micro-
power op amps, low power references (such as the AD589),
adjustment circuits, and the like.
The AD204 gets its power from an external clock driver, and
can handle loads on its isolated supply outputs of 2 mA for each
supply terminal (+7.5 V and –7.5 V) or 3 mA for a single loaded
output. Whenever the external load on either supply is more
than about 200
mA,
a 1
mF
tantalum capacitor should be used to
bypass each loaded supply pin to input common.
Up to 32 AD204s can be driven from a single AD246 (or equi-
valent) clock driver when the isolated power outputs of the
AD204s are loaded with less than 200
mA
each, at a worst-case
supply voltage of 14.25 V at the clock driver. The number of
AD204s that can be driven by one clock driver is reduced by
one AD204 per 3.5 mA of isolated power load current at 7.5 V,
distributed in any way over the AD204s being supplied by that
clock driver. Thus a load of 1.75 mA from +V
ISO
to –V
ISO
would
also count as one isolator because it spans 15 V.
It is possible to increase clock fanout by increasing supply volt-
age above the 14.25 V minimum required for 32 loads. One
additional isolator (or 3.5 mA unit load) can be driven for each
40 mV of increase in supply voltage up to 15 V. Therefore if the
minimum supply voltage can be held to 15 V – 1%, it is possible
to operate 32 AD204s and 52 mA of 7.5 V loads. Figure 14
shows the allowable combinations of load current and channel
count for various supply voltages.
50
AL
TOT
0mA
=
0.1”
GRID
Operation at Reduced Signal Swing.
Although the nominal
output signal swing for the AD202 and AD204 is
±
5 V, there
may be cases where a smaller signal range is desirable. When
that is done, the fixed errors (principally offset terms and output
noise) become a larger fraction of the signal, but nonlinearity is
reduced. This is shown in Figure 15.
0.025
0.020
NONLINEARITY – % span
0.015
0.010
0.005
0
0
1
3
2
OUTPUT SIGNAL SWING –
4
V
5
Figure 15. Nonlinearity vs. Signal Swing
PCB Layout for Multichannel Applications.
The pinout of
the AD204Y has been designed to make very dense packing
possible in multichannel applications. Figure 16a shows the
recommended printed circuit board (PCB) layout for the simple
voltage-follower connection. When gain-setting resistors are
present, 0.25" channel centers can still be achieved, as shown in
Figure 16b.
CHANNEL INPUTS
0
1
2
NUMBER OF AD204s DRIVEN
40
I
ISO
30
I
ISO
TAL
A TO
35m
=
TAL
A TO
70m
=
20
I
ISO
mA
= 80
I
ISO
TOTAL
CLK COM
10
OPERATION IN THIS REGION EXCEEDS
4mA LOAD LIMIT PER AD204
CLK
0
14.25
OUT COM
14.50
14.75
MINIMUM SUPPLY VOLTAGE
15.0
Figure 14. AD246 Fanout Rules
CHANNEL OUTPUTS
TO MUX
Figure 16a.
(NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to
Page 3 for proper DIP package pinout.)
–8–
REV. D
AD [ ANALOG DEVICES ]
