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AD420 Datasheet(PDF) 9 Page - Analog Devices

Part No. AD420
Description  Serial Input 16-Bit 4 mA-20 mA, 0 mA-20 mA DAC
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD420 Datasheet(HTML) 9 Page - Analog Devices

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REV. F
–9–
AD420
MICROPROCESSOR INTERFACE SECTION
AD420-TO-MC68HC11 (SPI BUS) INTERFACE
The AD420 interface to the Motorola SPI (Serial Peripheral
Interface) is shown in Figure 10. The MOSI, SCK, and
SS pins
of the HC11 are respectively connected to the DATA IN,
CLOCK, and LATCH pins of the AD420. The majority of the
interfacing issues are done in the software initialization. A typi-
cal routine such as the one shown below begins by initializing
the state of the various SPI data and control registers.
INIT
LDAA #$2F
;
SS = 1; SCK = 0; MOSI = 1
STAA
PORTD
;SEND TO SPI OUTPUTS
LDAA #$38
;
SS, SCK, MOSI = OUTPUTS
STAA
DDRD
;SEND DATA DIRECTION INFO
LDAA #$50
;DABL INTRPTS, SPI IS MASTER & ON
STAA
SPCR
;CPOL = 0, CPHA = 0, 1MHZ BAUDRATE
NEXTPT LDAA MSBY
;LOAD ACCUM W/UPPER 8 BITS
BSR
SENDAT ;JUMP TO DAC OUTPUT ROUTINE
JMP
NEXTPT ;INFINITE LOOP
SENDAT LDY
#$1000
;POINT AT ON-CHIP REGISTERS
BCLR
$08,Y,$20 ;DRIVE
SS (LATCH) LOW
STAA
SPDR
;SEND MS-BYTE TO SPI DATA REG
WAIT1
LDAA SPSR
;CHECK STATUS OF SPIE
BPL
WAIT1
;POLL FOR END OF X-MISSION
LDAA LSBY
;GET LOW 8 BITS FROM MEMORY
STAA
SPDR
;SEND LS-BYTE TO SPI DATA REG
WAIT2
LDAA SPSR
;CHECK STATUS OF SPIE
BPL
WAIT2;
;POLL FOR END OF X-MISSION
BSET
$08,Y,$20 ;DRIVE
SS HIGH TO LATCH DATA
RTS
The SPI data port is configured to process data in 8-bit bytes.
The most significant data byte (MSBY) is retrieved from
memory and processed by the SENDAT routine. The
SS pin is
driven low by indexing into the PORTD data register and clear
Bit 5. The MSBY is then sent to the SPI data register where it is
automatically transferred to the AD420 internal shift resister.
The HC11 generates the requisite eight clock pulses with data
valid on the rising edges. After the MSBY is transmitted, the
least significant byte (LSBY) is loaded from memory and trans-
mitted in a similar fashion. To complete the transfer, the
LATCH pin is driven high when loading the complete 16-bit
word into the AD420.
DATA IN
CLOCK
LATCH
MOSI
SCK
SS
AD420
68HC11
Figure 10. AD420-to-68HC11 (SPI) Interface
AD420-TO-MICROWIRE INTERFACE
The flexible serial interface of the AD420 is also compatible
with the National Semiconductor MICROWIRE interface. The
MICROWIRE interface is used in microcontrollers such as the
COP400 and COP800 series of processors. A generic interface
to use the MICROWIRE interface is shown in Figure 11. The
G1, SK, and SO pins of the MICROWIRE interface are respec-
tively connected to the LATCH, CLOCK, and DATA IN pins
of the AD420.
DATA IN
CLOCK
LATCH
SO
SK
AD420
MICROWIRE
G1
Figure 11. AD420-to-MICROWIRE Interface
EXTERNAL BOOST FUNCTION
The external boost transistor reduces the power dissipated in
the AD420 by reducing the current flowing in the on-chip
output transistor (dividing it by the current gain of the external
circuit). A discrete NPN transistor with a breakdown voltage,
BVCEO, greater than 32 V can be used as shown in Figure 12.
BOOST
IOUT
RLOAD
AD420
19
18
MJD31C
OR
2N3053
1k
0.022 F
Figure 12. External Boost Configuration
The external boost capability has been developed for those users
who may wish to use the AD420, in the SOIC package, at the
extremes of the supply voltage, load current, and temperature
range. The PDIP package (because of its lower thermal resis-
tance) will operate safely over the entire specified voltage, tem-
perature, and load current ranges without the boost transistor.
The plot in Figure 13 shows the safe operating region for both
package types. The boost transistor can also be used to reduce
the amount of temperature induced drift in the part. This will
minimize the temperature induced drift of the on-chip voltage
reference, which improves drift and linearity.
VCC
28V
20V
12V
4V
–60
–40
–20
0
20
40
60
80
100
TEMPERATURE – C
25V
WHEN USING SOIC PACKAGED DEVICES, AN
EXTERNAL BOOST TRANSISTOR IS REQUIRED
FOR OPERATION IN THIS AREA
AD420 OR AD420-32
32V
Figure 13. Safe Operating Region


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