AD202/AD204
should be made large to minimize inductance and resistance;
ideally, output LO should be directly connected to a ground
plane which serves as measurement common.
Current spikes can be greatly reduced by connecting a small in-
ductance (68
µH–100 µH)
in series with the clock pin of each
AD204. Molded chokes such as the Dale IM-2 series, with dc
resistance of about 5
Ω,
are suitable.
Up to 32 AD204s can be driven from a single AD246 (or
equivalent) clock driver when the isolated power outputs of the
AD204s are loaded with less than 200
µA
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 ad-
ditional 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.
Figure 13. Output Filter Circuit Showing Proper Grounding
Using Isolated Power.
Both the AD202 and the AD204 pro-
vide
±
7.5 V power outputs referenced to input common. These
may be used to power various accessory circuits which must
operate at the input common-mode level; the input zero adjust-
ment 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
µA
total from ei-
ther or both outputs) is much more limited in current capacity
than that of the AD204, but it is sufficient for operating mi-
cropower 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
µA,
a 1
µF
tantalum capacitor should be
used to bypass each loaded supply pin to input common.
Figure 14. AD246 Fanout Rules
(Circuit figures shown on this page are for SIP style pack-
ages. Refer to page 3 for proper DIP package pinout.)
–8–
REV. B
AD [ ANALOG DEVICES ]
