Datasheet AD202, AD204 (Analog Devices) - 6

AD202/AD204 Adjustments.
When gain and zero adjustments are needed, the
5k GAIN AD202
circuit details will depend on whether adjustments are to be made
OR
at the isolator input or output, and (for input adjustments) on
47.5k AD204 RS
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,
VS
and because gain adjustment is most easily done as part of the
200
gain-setting network. Input adjustments are also to be preferred when the pots will be near the input end of the isolator (to mini-
50k +7.5
mize common-mode strays). Adjustments on the output side
100k
might be used if pots on the input side would represent a hazard
–7.5 ZERO
due to the presence of large common-mode voltages during adjustment. Figure 8b. Adjustments for Summing or Current Input Figure 8a shows the input-side adjustment connections for use Figure 9 shows how zero adjustment is done at the output by with the noninverting connection of the input amplifier. The taking advantage of the semi-floating output port. The range of zero adjustment circuit injects a small adjustment voltage in series this adjustment will have to be increased at higher gains; if that with the low side of the signal source. (This will not work if the is done, be sure to use a suitably stable supply voltage for the source has another current path to input common or if current pot circuit. flows in the signal source LO lead). Since the adjustment volt- There is no easy way to adjust gain at the output side of the age is injected ahead of the gain, the values shown will work for isolator itself. If gain adjustment must be done on the output any gain. Keep the resistance in series with input LO below a side, it will have to be in a following circuit such as an output few hundred ohms to avoid CMR degradation. buffer or filter.
5k GAIN AD202 AD202 OR OR 47.5k AD204 AD204 2k VO +15V VS RG 50k 100k ZERO 200 0.1 F 200 +7.5 50k –15V 100k –7.5 ZERO
Figure 9. Output-Side Zero Adjustment Figure 8a. Adjustments for Noninverting Connection of Op Amp
Common-Mode Performance.
Figures 10a and 10b show how the common-mode rejection of the AD202 and AD204 Also shown in Figure 8a is the preferred means of adjusting the varies with frequency, gain, and source resistance. For these gain-setting network. The circuit shown gives a nominal RF of isolators, the significant resistance will normally be that in the 50 kW, and will work properly for gains of ten or greater. The path from the source of the common-mode signal to IN COM. adjustment becomes less effective at lower gains (its effect is The AD202 and AD204 also perform well in applications re- halved at G = 2) so that the pot will have to be a larger fraction quiring rejection of fast common-mode steps, as described in of the total RF at low gain. At G = 1 (follower) the gain cannot the Applications section. be adjusted downward without compromising input resistance; it is better to adjust gain at the signal source or after the output.
180
Figure 8b shows adjustments for use with inverting input cir-
G = 100 G = 1
cuits. The zero adjustment nulls the voltage at the summing
160
node. This method is preferable to current injection because it is
RLO = 0 140
less affected by subsequent gain adjustment. Gain adjustment is again done in the feedback; but in this case it will work all the
R 120 LO = 500
way down to unity gain (and below) without alteration.
RLO = 0 CMR – dB 100 R 80 LO = 10k RLO = 10k 60 4010 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 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS AD246–SPECIFICATIONS AD246 Pin Designations PIN DESIGNATIONS ORDERING GUIDE DIFFERENCES BETWEEN THE AD202 AND AD204 INSIDE THE AD202 AND AD204 USING THE AD202 AND AD204 Powering the AD204 AD246 Clock Drive Input Configurations Adjustments Common-Mode Performance Dynamics and Noise Using Isolated Power Operation at Reduced Signal Swing PCB Layout for Multichannel Applications Synchronization APPLICATIONS EXAMPLES Low Level Sensor Inputs Process Current Input with Offset High Compliance Current Source Motor Control Isolator. Floating Current Source/Ohmmeter Photodiode Amplifier OUTLINE DIMENSIONS Revision History