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AD204K

Low Cost, Miniature Isolation Amplifiers

Analog Devices 

AD202/AD204

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 adjustments) 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 mini-

mize 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 adjustment volt-

age is injected ahead of the gain, the values shown will work for

any gain. Keep the resistance in series with input LO below a

few hundred ohms to avoid CMR degradation.

47.5k⍀

5k⍀

GAIN

2k⍀

VS

RG

AD202

OR

AD204

200⍀

50k⍀

100k⍀

ZERO

+7.5

–7.5

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 kW, 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.

5k⍀

GAIN

RS

47.5k⍀

AD202

OR

AD204

VS

200⍀

50k⍀

100k⍀

ZERO

+7.5

–7.5

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.

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

isolator 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.

AD202

OR

AD204

VO

+15V

200⍀

50k⍀

100k⍀

ZERO

0.1␮F

–15V

Figure 9. Output-Side Zero Adjustment

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

how the common-mode rejection of the AD202 and AD204

varies with frequency, gain, and source resistance. For these

isolators, the significant resistance will normally be that in the

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

The AD202 and AD204 also perform well in applications re-

quiring rejection of fast common-mode steps, as described in

the Applications section.

180

G = 100

160

G=1

140

RLO = 0⍀

120

RLO = 500⍀

100

RLO = 0⍀

80

RLO = 10k⍀

60

RLO = 10k⍀

40

10 20

50 60 100 200 500 1k 2k

5k

FREQUENCY – Hz

Figure 10a. AD204

(NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to

Page 3 for proper DIP package pinout.)

–6–

REV. D

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