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AD694BRZ датащи(PDF) 11 Page - Analog Devices |
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AD694BRZ датащи(HTML) 11 Page - Analog Devices |
11 / 16 page AD694 REV. B –11– Figure 13. Digital to 4–20 mA Interface Using a Current Steering DAC Figure 14. Single-Supply Digital Input to 4–20 mA Output code dependent, and the response time of the circuit will be de- termined by the reaction of the voltage reference. The supply voltage to the AD7541A should be kept close to 15 V. If VS is reduced significantly from 15 V the differential nonlinearity of the DAC will increase and the linearity will be degraded. In some applications it is desirable to have some underrange and overrange in the 4–20 mA output. For example, assume an over and under range capability of ±5% of span is needed, then the output current range corresponding to the full scale of the DAC is 3.2 mA to 20.8 mA. To accomplish this, the span of the AD694 would be increased 10% to 17.6 mA by adding a nonin- verting gain of 1.1 to the buffer amplifier. The 4 mA offset would then be reduced by 0.8 mA, by utilizing the adjustment scheme explained in Adjusting 4 mA Zero section. Then a digi- tal input from all zero code to full scale would result in an out- put current of 3.2 mA to 20.8 mA. LOW COST SENSOR TRANSMITTER Sensor bridges typically output differential signals in the 10 mV to 100 mV full-scale range. With an AD694, a dual op amp, and some resistors, an instrumentation amplifier front end can be added which easily handles these types of low level signals. The traditional 3 op amp instrumentation amplifier is built us- ing an AD708 dual op amp for the front end, and the AD694’s buffer amplifier is used for the subtractor circuit, as shown in Figure 15. The AD694’s 2 V reference is used to provide a “ground” of 2 V that ensures proper operation of the in amp over a wide common mode range. The reference pin of the subtractor circuit is tied to the 2 V reference (point C). A 2 k Ω pull-down resistor ensures that the voltage reference will be able to sink any subtractor current. The 2 V FS (Pin 4) is attached to the 2 V reference; this offsets the input range of the V/I con- verter 2 volts positive, to match the “ground” of the in amp. The AD694 will now output a 4–20 mA output current for a 0 V to 2 V differential swing across VA. The gain of the in amp front end is adjusted so that the desired full-scale input signal at VIN results in a VA of 2 V. For example a sensor that has a 100 mV full scale will require a gain of 20 in the front end. The gain is determined according to the equation: G = [2RS/Rg] + 1 |
Аналогичный номер детали - AD694BRZ |
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Аналогичное описание - AD694BRZ |
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