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AD202KN 데이터시트(PDF) 6 Page - Analog Devices |
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AD202KN 데이터시트(HTML) 6 Page - Analog Devices |
6 / 12 page REV. D AD202/AD204 –6– 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. AD202 OR AD204 47.5k VS 5k GAIN RG 200 +7.5 –7.5 100k ZERO 2k 50k 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 k W, 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. AD202 OR AD204 RS 47.5k VS 5k GAIN 200 50k +7.5 –7.5 100k ZERO 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 200 50k 100k ZERO 0.1 F +15V –15V VO 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. FREQUENCY – Hz 180 10 160 140 120 100 80 60 40 20 50 60 100 200 500 1k 2k 5k G = 100 G = 1 R LO = 10k R LO = 500 R LO = 0 R LO = 0 R LO = 10k Figure 10a. AD204 (NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to Page 3 for proper DIP package pinout.) |
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