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AD202J Arkusz danych(PDF) 10 Page - Analog Devices |
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AD202J Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 12 page REV. B –10– AD202/AD204 Process Current Input with Offset. Figure 18 shows an iso- lator receiver which translates a 4-20 mA process current signal into a 0 V to +10 V output. A 1 V to 5 V signal appears at the isolator’s output, and a –1 V reference applied to output LO provides the necessary level shift (in multichannel applications, the reference can be shared by all channels). This technique is often useful for getting offset with a follower-type output buffer. 4 37 38 3 2 –15V +15V 15k 10k 0 TO +10V 6 4 7 1 2 250 Ω AD202 OR AD204 1V TO 5V 3 4-20mA –1V TO ADDITIONAL CHANNELS 1k –15V AD589 237 Ω 6.8k Figure 18. Process Current Input Isolator with Offset The circuit as shown requires a source compliance of at least 5 V, but if necessary that can be reduced by using a lower value of current-sampling resistor and configuring the input amplifier for a small gain. High-Compliance Current Source. In Figure 19, an isolator is used to sense the voltage across current-sensing resistor R to allow direct feedback control of a high-voltage transistor or FET used as a high-compliance current source. Since the isolator has virtually no response to dc common-mode voltage, the closed- loop current source has a static output resistance greater than 10 14 Ω even for output currents of several mA. The output cur- rent capability of the circuit is limited only by power dissipation in the source transistor. 4 3 1 AD202 OR AD204 38 37 ~ LOAD IL = VC RS –10V TO +250V 100k 470pF ~ +15V +5V REF 20k VC ~ –15V 10k 1k MPS U10 RS 1k Ω 2 Figure 19. High-Compliance Current Source Synchronization. Since AD204s operate from a common clock, synchronization is inherent. AD202s will normally not in- teract to produce beat frequencies even when mounted on 0.25- inch centers. Interaction may occur in rare situations where a large number of long, unshielded input cables are bundled to- gether and channel gains are high. In such cases, shielded cable may be required or AD204s can be used. APPLICATIONS EXAMPLES Low-Level Sensor Inputs. In applications where the output of low-level sensors such as thermocouples must be isolated, a low drift input amplifier can be used with an AD204, as shown in Figure 17. A three-pole active filter is included in the design to get normal-mode rejection of frequencies above a few Hz and to provide enhanced common-mode rejection at 60 Hz. If offset adjustment is needed, it is best done at the trim pins of the OP07 itself; gain adjustment can be done at the feedback resistor. 3 1 0.15µF 33 32 4 470k 470k CLK RET CLK 37 6 2 0.039µF 38 VO = VI x 1 + 50k RG () 5 1µF 1µF RG 49.9k 4 7 2 3 6 39k 220M OPTIONAL OPEN INPUT DETECTION AD204 +7.5V –7.5V HI LO 1µF AD OP-07 Figure 17. Input Amplifier & Filter for Sensor Signals Note that the isolated supply current is large enough to mandate the use of 1 µF supply bypass capacitors. This circuit can be used with an AD202 if a low-power op amp is used instead of the OP07. (Circuit figures shown on this page are for SIP style pack- ages. Refer to page 3 for proper DIP package pinout.) |
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