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AD760AQ Datenblatt(PDF) 10 Page - Analog Devices |
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AD760AQ Datenblatt(HTML) 10 Page - Analog Devices |
10 / 12 page –10– AD760 REV. A Byte Mode Operation is enabled by setting SER high, which configures DB0–DB7 as data inputs. In this mode HBE and LBE are used to identify the data as either the high byte or the low byte of the 16-bit word. The user can load the data in either order into the first rank latch using the rising edge of the CS signal as shown in Figure 1a. The status of Pin 17 when CLR is strobed determines whether the AD760 clears to unipolar or bipolar zero. (But it cannot be hardwired to the desired state, as in the serial mode.) NOTE: CS is edge triggered. HBE, LBE, CLR, SER, CAL, and LDAC are level triggered. USING THE OUTPUT MULTIPLEXER The onboard multiplexer allows the user to isolate the load from the voltage variations at VOUT during calibration. To minimize the glitch-impulse at MUXOUT, the multiplexer input, MUXIN, should be tied to a voltage equal to the DAC’s negative full-scale voltage. Since the DAC is loaded with the contents of its first-rank latch before completing calibration, the DAC should be programmed to negative full scale before calibrating. This will minimize the voltage excursions of MUXOUT at the beginning and end of calibration. If the glitch-impulse at the beginning of calibration is not important, yet the user wants to minimize the recovery time at MUXOUT, MUXIN should be set to the voltage that corresponds to the data in the first-rank latch before calibration is initiated. The multiplexer series on-resistance limits its load-drive capability. To attain 16-bit linearity, MUXOUT must be buffered with a suitable op amp. The amplifier open loop-gain and common- mode rejection contribute to gain error whereas the linearity of these parameters affect the relative accuracy (or integral nonlin- earity). In general, the amplifier linearity is not specified so its effects must be determined empirically. Using the AD707, as shown in Figure 9, the overall linearity error is within 0.5 LSB. The AD707C/T initial voltage offset and its temperature coeffi- cient will not contribute more than 0.1 LSB to the Bipolar Zero Error over the entire operating temperature range. The settling time to 1/2 LSB is typically 100 µs for a 20 V step. For applica- tions that require faster settling, the AD820 can be used to attain full-scale settling to within a 1/2 LSB in 20 µs. The addi- tional linearity error from the AD820 will be no more than 0.25 LSB. 100pF +VCC 0.1µF OUTPUT AD707 OR AD820 2 7 6 3 4 MUXOUT 22 23 28 27 1 4 MUXIN AGND VOUT 24 3 SPAN/ BIP OFF CALOK +VCC 1k Ω 0.1µF AD760 –VEE –VEE Figure 9. Buffering the AD760 Internal MUX USING AN EXTERNAL MULTIPLEXER An external multiplexer like the ADG419 allows the user to minimize the glitch impulse when holding the output to any predetermined voltage during calibration. The ADG419 can be used with a high speed op amp like the AD829, as shown in Fig- ure 10, to attain the fastest possible settling time while main- taining 16-bit linearity. The settling time to 1/2 LSB for a 20 V step is typically 10 µs. AD760 TO MC68HC11 (SPI* BUS) INTERFACE The AD760 interface to the Motorola SPI (serial peripheral in- terface) is shown in Figure 11. The MOSI, SCK, and SS pins of the HC11 are respectively connected to the SIN, CS and LDAC pins of the AD760. The majority of the interfacing issues are taken care of in the software initialization. A typical routine such as the one shown below begins by initializing the state of the various SPI data and control registers. The most significant data byte (MSBY) is then retrieved from memory and processed by the SENDAT subroutine. The SS pin is driven low by indexing into the PORTD data register and clearing Bit 5. The MSBY is then sent to the SPI data register where it is automatically transferred to the AD760. *SPI is a registered trademark of Motorola. MUXOUT 22 23 28 1 4 MUXIN AGND VOUT 24 3 SPAN/ BIP OFF CALOK +VCC AD760 –VEE 1nF +VCC 0.1µF OUT 2 7 6 3 4 1k Ω 0.1µF –VEE 60pF ADG419 8 2 1 6 27 5 4 +VCC 7 –VEE AD829 Figure 10. Using the AD760 with an External MUX |
Ähnliche Teilenummer - AD760AQ |
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Ähnliche Beschreibung - AD760AQ |
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