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AD421 Fiches technique(PDF) 10 Page - Analog Devices |
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AD421 Fiches technique(HTML) 10 Page - Analog Devices |
10 / 14 page AD421 –10– REV. C A capacitor of 0.01 µF connected between COMP and DRIVE is required to stabilize the feedback loop formed with the regulator op amp and the external pass transistor. An external snubber circuit of 1 k Ω and 1000 pF is required between the DRIVE pin and COM and a 0.1 µF cap between COMP and DRIVE to stabilize the feedback loop formed by the regulator op amp and the external pass transistor. The internal 2.5 V reference on the AD421 is used as the refer- ence for the AD421 and this has to be decoupled with a 4.7 µF capacitor for compensation and stability purposes. The sigma- delta DAC on the part consists of a second order modulator followed by a continuous time filter. The resistors for each of the filter sections are on-chip while the capacitors are external on the C1 to C3 pins. To meet the specified full-scale settling on the part, low dielectric absorption capacitors (NPO) are required. Suitable values for these capacitors are C1 = C2 = 0.01 µF, and C3 = 0.0033 µF. The digital interface on the AD421 consists of just three wires: DATA, CLOCK and LATCH. The interface connects directly to the serial ports of commonly-used microcontrollers without the need for any external glue logic. Data is loaded into an input shift register on the rising edge of the CLOCK signal and is transferred to the DAC latch on the rising edge of the LATCH signal. Reduce Power Load on External FET Figure 12 shows a circuit where an external NPN transistor is added to reduce the power loading on the FET. The FET will supply the VCC and an external high voltage NPN bipolar tran- sistor can carry the BOOST current. The BOOST pin sinks the necessary current from the loop so that the current flowing into BOOST plus the current flowing into COM is equal to the programmed loop current. The external NPN transistor reduces the external power load that the FET has to carry to less than 750 µA if no other components share the V CC line and to less than 4 mA in applications that share the same VCC line as the AD421. 1.21V 112.5k 134k 75k 121k VCC LV 2.2 F COM VCC TO EXTERNAL CIRCUITRY DN25D DRIVE COMP 0.01 F 1k 1000pF LOOP(+) BANDGAP REFERENCE AD421 BC639/BC337 BOOST 40 80k LOOP RTN LOOP(–) Figure 12. External NPN Transistor Reduces Power Load on FET Smart Transmitter The AD421 is intended for use in 4 mA to 20 mA smart trans- mitters. A smart transmitter is a system that incorporates a microprocessor system which is used for linearization and communication. Figure 13 shows a block diagram of a typical smart transmitter. In this example, the transmitter does not have any digital communication capabilities. 4mA TO 20mA MEASUREMENT CIRCUIT MICRO- PROCESSOR D/A CONVERTER A/D CONVERTER MEMORY SENSORS Figure 13. Typical Smart Transmitter Figure 14 shows a typical smart transmitter application circuit using the AD421. The sensor voltage to be measured at the transmitter is con- verted using a high resolution sigma-delta converter such as the AD7714 or AD7715. These devices have an on-board PGA which can provide gains on the analog front end from 1 to 128. This allows for an analog input range as low as 10 mV which allows the transducer to be connected directly to the ADC. The AD7714/AD7715 have digital calibration techniques which are used to eliminate gain and offset errors. In addition, back- ground calibration techniques are provided whereby the part continually calibrates itself and the user does not have to worry about issuing periodic calibration commands to remove effects of time and temperature drift. In normal operation the microprocessor reads the data from the AD7714/AD7715. After the data is processed by the micro- controller, the data is transferred from the serial port of the processor to the AD421 for transmission over the 4 to 20 mA loop back to the control center. The AD421 regulates the loop voltage to create power for the rest of the transmitter circuitry. In Figure 14, the derived VCC voltage is 3.3 V which is achieved by connecting the LV pin to VCC through 0.01 µF. REF OUT2 provides the reference volt- age for the AD421 itself while REF OUT1 provides the refer- ence voltage for the AD7714 /AD7715. |
Numéro de pièce similaire - AD421 |
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Description similaire - AD421 |
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