Motore di ricerca datesheet componenti elettronici |
|
AD421 Scheda tecnica(PDF) 7 Page - Analog Devices |
|
AD421 Scheda tecnica(HTML) 7 Page - Analog Devices |
7 / 14 page AD421 –7– REV. C Reference Section The AD421 contains an on-chip 1.21 V bandgap reference which is used as part of the voltage regulator loop. A bandgap reference is also used to generate two references voltages which are available for use external to the AD421. Figure 5 shows the reference section of the AD421. The REF OUT1 pin provides a buffered +1.25 V reference voltage which can supply up to 0.5 mA of external current. The REF OUT2 pin provides a +2.5 V reference voltage which is also capable of providing 0.5 mA of external current. To use the AD421 with its own reference, simply connect the REF OUT2 pin to the REF IN pin of the device. Alternatively, the part can be used with an external reference by connecting the external reference between REF IN and COM. When REF OUT1 and REF OUT2 are used in application circuits, external 4.7 µF capacitors are required on the reference pins to provide compensation and ensure stable operation of the references. These capacitors can be omitted if the internal refer- ences are not required. 1.21V 112.5k 134k 75k 121k VCC LV 4.7 F DRIVE 2.5V BANDGAP REFERENCE AD421 50k REF OUT2 (2.5V) 50k 4.7 F REF OUT1 (1.25V) Figure 5. Reference Section REF OUT2 is sensed internally, and if more than 0.5 mA is drawn externally from this reference, the chip goes into a power on reset state. In this state the sigma-delta DAC is disabled, the internal oscillator is stopped and the input data latch is cleared. REF OUT1 has limited current sinking capability. If REF OUT1 is required to sink current, a resistive load of 100 k Ω to COM should be added in addition to the 4.7 µF capacitor. USING THE AD421 The AD421 can be programmed for normal 4 mA to 20 mA operation or for alarm current operation. For normal operation, the coding is 16-bit straight (natural) binary over an output current range of 4 mA to 20 mA. For alarm current operation, the coding is also straight binary but with 17 bits of resolution over twice the span, 0 mA to 32 mA, although the part should not be programmed outside the range of 3.5 mA to 24 mA. To determine whether data written to the part is normal 4 mA to 20 mA data or alarm current data, the number of clock pulses between two successive LATCH pulses are counted. If the num- ber of pulses is 0–16 (modulo 32), it chooses normal mode; if it is 17–31 (modulo 32), it chooses alarm current range. 4 mA to 20 mA Coding Table II shows the ideal input-code-to-output-current relation- ship for normal operation of the AD421. The output current values shown assume a REF IN voltage of +2.5 V. With a REF IN of +2.5 V, 1 LSB = 16 mA/65,536 = 244 nA. Figure 6 shows a timing diagram for programming the AD421 for normal 4 mA to 20 mA operation, the AD421 outputting a current AD421 BOOST 40 80k LOOP RTN SWITCHED CURRENT SOURCES Figure 4. Current Amplifier The BOOST pin is normally tied to the VCC pin. As the DAC input code varies from all zeros to full scale, the output current from the NPN transistor and thus the total loop current varies from 4 mA to 20 mA. With BOOST and VCC tied together, the external FET (DN25D) has to supply the full range of loop current (4 mA to 20 mA). Digital Interface 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 MSB first 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. The timing diagrams for the serial interface are shown in Figure 1 and Figure 2. The data to be loaded to the AD421’s input shift register takes two forms; normal 4 mA to 20 mA data or alarm current data. The first form is where the AD421 operates over its normal 4 mA to 20 mA output range with 16 bits of resolution between these endpoints. The second form allows the user to program a current value outside this range as an indication from the trans- mitter than there is a problem with the transducer. The AD421 counts the number of clock pulses which it receives between LATCH signals as a means of determining whether the data clocked in is 4 mA to 20 mA data or alarm current data. If there are 16 rising clock edges between successive LATCH pulses, then the data being loaded to the input shift register is assumed to be normal 4 mA to 20 mA data. On the rising edge of the LATCH signal, the input shift register data is transferred to the DAC latch in a 16-bit parallel transfer. In this case, the 16 bits of data in the DAC latch program the output current between 4 mA for all 0s and 20 mA for all 1s (see Table II). Data transferred to the AD421 should be MSB first. If there are more than 16 clock pulses between successive LATCH pulses, then the data being loaded to the input shift register is assumed to be alarm current data. In this case, the AD421 accepts 17 bits of data into its shift register. For situa- tions where there are more than 17 clocks in the serial write operation (for example, 24 clocks in a 3 × 8-bit transfer from the serial port of a microcontroller) the AD421 simply accepts the last 17 bits of the serial write operation. Data transferred in this serial write operation is LSB last (i.e., the MSB is loaded on the 17th rising clock edge prior to the LATCH pulse). On the rising edge of the LATCH signal, the input shift register data is trans- ferred to the DAC latch in a 17-bit parallel transfer. In this case, the 17 bits of data in the DAC latch program the output current between 0 mA for all 0s and 32 mA for all 1s (see Table III). However, in practice the AD421 cannot reliably produce a current less than 3.5 mA or more than 24 mA. |
Codice articolo simile - AD421_15 |
|
Descrizione simile - AD421_15 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEETIT.COM |
Lei ha avuto il aiuto da alldatasheet? [ DONATE ] |
Di alldatasheet | Richest di pubblicita | contatti | Privacy Policy | scambio Link | Ricerca produttore All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |