AD204J(1994) Ver la hoja de datos (PDF) - Analog Devices

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AD204J
(Rev.:1994)

Low Cost, Miniature Isolation Amplifiers

Analog Devices 

AD202/AD204

work for any gain. Keep the resistance in series with input LO

below a few hundred ohms to avoid CMR degradation.

There is no easy way to adjust gain at the output side of the iso-

lator itself. If gain adjustment must be done on the output side,

it will have to be in a following circuit such as an output buffer

or filter.

Figure 8a. Adjustments for Noninverting Connection of

Op Amp

Also shown in Figure 8a is the preferred means of adjusting the

gain-setting network. The circuit shown gives a nominal RF of

50 kΩ, and will work properly for gains of ten or greater. The

adjustment becomes less effective at lower gains (its effect is

halved at G = 2) so that the pot will have to be a larger fraction

of the total RF at low gain. At G = 1 (follower) the gain cannot

be adjusted downward without compromising input resistance;

it is better to adjust gain at the signal source or after the output.

Figure 8b shows adjustments for use with inverting input cir-

cuits. The zero adjustment nulls the voltage at the summing

node. This method is preferable to current injection because it is

less affected by subsequent gain adjustment. Gain adjustment is

again done in the feedback; but in this case it will work all the

way down to unity gain (and below) without alteration.

Figure 9. Output-Side Zero Adjustment

Common-Mode Performance. Figures 10a and 10b show

how the common-mode rejection of the AD202 and AD204 var-

ies with frequency, gain, and source resistance. For these isola-

tors, the significant resistance will normally be that the path

from the source of the common-mode signal to IN COM. The

AD202 and AD204 also perform well in applications requiring

rejection of fast common-mode steps, as described in the Appli-

cations section.

Figure 8b. Adjustments for Summing or Current Input

Figure 9 shows how zero adjustment is done at the output by

taking advantage of the semi-floating output port. The range of

this adjustment will have to be increased at higher gains; if that

is done, be sure to use a suitably stable supply voltage for the

pot circuit.

Figure 10a. AD204

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

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

–6–

REV. B

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