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AD7846BP

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AD7846
Rev. G | Page 15 of 24
POSITION MEASUREMENT APPLICATION
Figure 25 shows the AD7846 in a position measurement applica-
tion using an linear variable displacement transducer (LVDT),
an AD630 synchronous demodulator and a comparator to make
a 16-bit LVDT-to-digital converter. The LVDT is excited with a
fixed frequency and fixed amplitude sine wave (usually 2.5 kHz,
2 V p-p). The outputs of the secondary coil are in antiphase and
their relative amplitudes depend on the position of the core in the
LVDT. The AD7846 output interpolates between these two inputs
in response to the DAC input code. The AD630 is set up so that
it rectifies the DAC output signal. Thus, if the output of the DAC is
in phase with the V
REF+
input, the inverting input to the compara-
tor is positive, and if it is in phase with V
REF−
, the output is nega-
tive. By turning on each bit of the DAC in succession starting
with the MSB and deciding to leave it on or turn it off based on
the comparator output, a 16-bit measurement of the core position
is obtained.
AD7846*
LVDT
DB0DB15
R1
100kΩ
C1
1µF
PROCESSOR DATA BUS
SIGNAL
GROUND
TO
PROCESSOR PORT
*ADDITIONAL PINS OMITTED FOR CLARITY
x ASIN ω t
–(1–x) ASIN ω t
A
SIN ω t
AD630*
DGND
V
OUT
R
IN
V
REF+
V
REF–
6
20
7
5
8
10
13
10
9
16
3
08490-027
Figure 25. AD7846 in Position Measurement Application
AD7846
Rev. G | Page 16 of 24
MICROPROCESSOR INTERFACING
AD7846-TO-8086 INTERFACE
Figure 26 shows the 8086 16-bit processor interfacing to the
AD7846. The double buffering feature of the DAC is not used in
this circuit because
LDAC
is permanently tied to 0 V. AD0 to
AD15 (the 16-bit data bus) are connected to the DAC data bus
(DB0 to DB15). The 16-bit word is written to the DAC in one
MOV instruction and the analog output responds immediately.
In this example, the DAC address is 0xD000.
AD7846*
+5V
DATA BUS
CS
LDAC
CLR
R/W
DB0 TO DB15
16-BIT
LATCH
8086
ALE
DEN
RD
WR
AD0 TO AD15
ADDRESS
DECODE
ADDRESS BUS
08490-028
*LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 26. AD7846-to-8086 Interface Circuit
In a multiple DAC system, the double buffering of the AD7846
allows the user to simultaneously update all DACs. In Figure 27,
a 16-bit word is loaded to the input latches of each of the DACs
in sequence. Then, with one instruction to the appropriate
address,
CS4
(that is,
LDAC
) is brought low, updating all the
DACs simultaneously.
+5V
DATA BUS
*LINEAR CIRCUITRY OMITTED FOR CLARITY
16-BIT
LATCH
ADDRESS
DECODE
ADDRESS BUS
AD7846*
CS
LDAC
R/W
DB0 TO DB15
8086
ALE
DEN
RD
WR
AD0 TO AD15
CLR
CLR
+5V
CLR
+5V
AD7846*
CS
LDAC
R/W
DB0 TO DB15
AD7846*
CS
LDAC
R/W
DB0 TO DB15
08490-029
Figure 27. AD7846-to-8086 Interface: Multiple DAC System
AD7846-TO-MC68000 INTERFACE
Interfacing between the AD7846 and MC68000 is accomplished
using the circuit of Figure 28. The following routine writes data
to the DAC latches and then outputs the data via the DAC latch.
1
000 MOVE.W
#W,
D0
The desired DAC dat
a
,
W, is loaded into
Data Register 0. W
may be any value
between 0 and 65535
(decimal) or 0 and
FFFF (hexadecimal).
MOVE.W
D0,
$
E000
The data, W, is
transferred between
D0 and the DAC
register.
MOVE.W
TRAP
#
228,
D7
#14
Control is returned
to the System Monitor
using these two
instructions.
AD7846*
+5V
DATA BUS
CS
LDAC
CLR
R/W
MC68000
DS
DTACK
R/W
A1 TO A23
ADDRESS
DECODE
ADDRESS BUS
D0 TO D15 DB0 TO DB15
0
8490-030
*
LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 28. AD7846-to-MC68000 Interface
AD7846
Rev. G | Page 17 of 24
DIGITAL FEEDTHROUGH
In the preceding interface configurations, most digital inputs to
the AD7846 are directly connected to the microprocessor bus.
Even when the device is not selected, these inputs are constantly
changing. The high frequency logic activity on the bus can feed
through the DAC package capacitance to show up as noise on
the analog output. To minimize this digital feedthrough, isolate
the DAC from the noise source. Figure 29 shows an interface
circuit that isolates the DAC from the bus.
Note that to make use of the AD7846 readback feature using
the isolation technique of Figure 29, the latch needs to be
bidirectional.
AD7846*
+5V
DATA BUS
CS
LDAC
CLR
MICRO-
PROCESSOR
A1 TO A15
ADDRESS
DECODE
ADDRESS BUS
D0 TO D15 DB0 TO DB15
R/W R/W
2×
74LS245
B BUS A BUS
DIR G
08490-031
*LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 29. AD7846 Interface Circuit Using Latches to Minimize Digital Feedthrough
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AD7846BP

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Buy AD7846BP
Manufacturer:
Analog Devices Inc.
Description:
Digital to Analog Converters - DAC 16-Bit VOut CMOS
Lifecycle:
New from this manufacturer.
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