AD202/AD204

REV. B

–5–

The “noninverting” circuit of Figures 5 and 6 can also be used

to advantage when a signal inversion is needed: just interchange

either the input leads or the output leads to get inversion. This

approach retains the high input resistance of the “noninverting”

circuit, and at unity gain no gain-setting resistors are needed.

When the isolator is not powered, a negative input voltage of

more than about 2 V will cause an input current to flow. If the

signal source can supply more than a few mA under such condi-

tions, the 2 k

Ω resistor shown in series with IN+ should be used

to limit current to a safe value. This is particularly important

with the AD202, which may not start if a large input current is

present.

Figure 7 shows how to accommodate current inputs or sum cur-

rents or voltages. This circuit can also be used when the input

signal is larger than the

±5 V input range of the isolator; for

example, a

±50 V input span can be accommodated with R

COM is required for gains above five.

Figure 7. Connections for Summing or Current Inputs

Adjustments.

When gain and zero adjustments are needed, the

circuit details will depend on whether adjustments are to be

made at the isolator input or output, and (for input adjust-

ments) on the input circuit used. Adjustments are usually best

done on the input side, because it is better to null the zero

ahead of the gain, and because gain adjustment is most easily

done as part of the gain-setting network. Input adjustments are

also to be preferred when the pots will be near the input end of

the isolator (to minimize common-mode strays). Adjustments

on the output side might be used if pots on the input side would

represent a hazard due to the presence of large common-mode

voltages during adjustment.

Figure 8a shows the input-side adjustment connections for use

with the “noninverting” connection of the input amplifier. The

zero adjustment circuit injects a small adjustment voltage in

series with the low side of the signal source. (This will not work

if the source has another current path to input common or if

current flows in the signal source LO lead). Since the adjust-

ment voltage is injected ahead of the gain, the values shown will

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

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

AD204s are operated from a single AD246, an external bypass

capacitor should be used with a value of at least 1

µF for every

five isolators used. Place the capacitor as close as possible to the

clock driver.

Figure 4. Clock Driver

Input Configurations.

The AD202 and AD204 have been de-

signed to be very easy to use in a wide range of applications.

The basic connection for standard unity gain applications, useful

for signals up to

±5 V, is shown in Figure 5; some of the possible

variations are described below. When smaller signals must be

Figure 5. Basic Unity-Gain Application

handled, Figure 6 shows how to get gain while preserving a very

be kept above 20 k

Ω for best results. Whenever a gain of more

than five is taken, a 100 pF capacitor from FB to IN COM is

required. At lower gains this capacitor is unnecessary, but it will

not adversely affect performance if used.

Figure 6. Input Connections for Gain > 1