REV. D

AD202/AD204

–8–

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

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.

MINIMUM SUPPLY VOLTAGE

14.25

50

0

14.50

14.75

40

30

20

10

15.0

= 0mA

TOT

AL

= 35mA

TOT

AL

= 70mA

TOT

AL

= 80mA

OPERATION IN THIS REGION EXCEEDS

4mA LOAD LIMIT PER AD204

TOT

AL

Figure 14. AD246 Fanout Rules

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.

OUTPUT SIGNAL SWING –

V

0

0.025

0

2

3

0.020

0.015

0.010

0.005

5

14

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

01

2

0.1”

GRID

CLK COM

CLK

OUT COM

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.)