AD7849
Other Output Voltage Ranges
MICROPROCESSOR INTERFACING
In some cases, users may require output voltage ranges other
than those already mentioned. One example is systems which
need the output voltage to be a whole number of millivolts (i.e.,
1 mV, 2 mV, etc.,). If Figure 18 is used, then the LSB size is
125
µV.
This makes it possible to program whole millivolt val-
ues at the output. Table III shows the code table for Figure 18.
+15V
+5V
Microprocessor interfacing to the AD7849 is via a serial bus
which uses standard protocol compatible with DSP processors
and microcontrollers. The communications channel requires a
three-wire interface consisting of a clock signal, a data signal
and a synchronization signal. The AD7849 requires a 16-bit
data word with data valid on the falling edge of SCLK. For all
the interfaces, the DAC update may be done automatically
when all the data is clocked in or it may be done under control
of
LDAC.
Figures 20 to 24 show the AD7849 configured for interfacing to
a number of popular DSP processors and microcontrollers.
V
DD
8
1
R1
8.192V
V
REF+
V
CC
R
OFS
V
OUT
V
OUT
(0V TO +8.192V)
AD584
R2
4
AD7849-ADSP-2101/ADSP-2102 Interface
AD7849*
DGND
V
REF–
AGND
SIGNAL
GND
*
ADDITIONAL PINS
OMITTED FOR CLARITY
Figure 18. 0 V to 8.192 V Output Range
Table III. Code Table for Figure 18
Binary Number
in DAC Latch
MSB
LSB
1111
1000
0000
0000
0000
0000
1111
0000
0000
0000
0000
0000
1111
0000
0000
0000
0000
0000
1111
0000
1000
0100
0010
0001
Analog Output
(V
OUT
)
8.192 V (65535/65536) = 8.1919 V
8.192 V (32768/65536) = 4.096 V
8.192 V (8/65536) = 0.001 V
8.192 V (4/65536) = 0.0005 V
8.192 V (2/65536) = 0.00025 V
8.192 V (1/65536) = 0.000125 V
Figure 20 shows a serial interface between the AD7849 and the
ADSP-2101/ADSP-2102 DSP processor. The ADSP-2101/
ADSP-2102 contains two serial ports and either port may be
used in the interface. The data transfer is initiated by
TFS
going
low. Data from the ADSP-2101/ ADSP-2102 is clocked into the
AD7849 on the falling edge of SCLK. The DAC can be up-
dated by holding
LDAC
high while performing the write cycle.
TFS
must be taken high after the 16-bit write cycle.
LDAC
is
brought low at the end of the cycle and the DAC output is up-
dated. In the interface shown the DAC is updated using an ex-
ternal timer which generates an
LDAC
pulse. This could also be
done using a control or decoded address line from the proces-
sor. Alternatively, if the
LDAC
input is hardwired low the out-
put update takes place automatically on the 16th falling edge of
SCLK.
TIMER
ADSP-2101
ADSP-2102*
SCLK
DT
TFS
LDAC
SCLK
SDIN
SYNC
AD7849*
NOTE: Assumes 16-bit resolution; 1 LSB = 8.192 V/2
16
= 125
µV.
Generating
5 V Output Range From Single +5 V Reference
*ADDITIONAL PINS OMITTED FOR CLARITY
The diagram below shows how to generate a
±
5 V output range
when using a single +5 V reference. V
REF–
is connected to 0 V
and R
OFS
is connected to V
REF+
. The +5 V reference input is
applied to these pins. With all 0s loaded to the DAC, the non-
inverting terminal of the output stage amplifier is at 0 V and
V
OUT
is simply the inverse of V
REF+
. With all 1s loaded to the
DAC, the noninverting terminal of the output stage amplifier is
at 5 V and so V
OUT
is also at 5 V.
+15V
+5V
Figure 20. AD7849 to ADSP-2101/ADSP-2102 Interface
AD7849-DSP56000 Interface
2
8
6
V
DD
R
OFS
V
REF+
R1
10kΩ
V
CC
V
OUT
AD586
C1
1nF
5
4
V
OUT
(–5V TO +5V)
A serial interface between the AD7849 and the DSP56000 is
shown in Figure 21. The DSP56000 is configured for Normal
Mode Asynchronous operation with Gated Clock. It is also set
up for a 16-bit word with SCK and SC2 as outputs and the FSL
control bit set to a “0”. SCK is internally generated on the
DSP56000 and applied to the AD7849 SCLK input. Data from
the DSP56000 is valid on the falling edge of SCK. The SC2
output provides the framing pulse for valid data. This line must
be inverted before being applied to the
SYNC
input of the
AD7849.
In this interface an
LDAC
pulse generated from an external
timer is used to update the outputs of the DACs. This update
can also be produced using a bit programmable control line
from the DSP56000.
AD7849*
DGND
V
REF–
V
SS
AGND
SIGNAL GND
*
ADDITIONAL PINS
OMITTED FOR CLARITY
–15V
Figure 19. Generating
±
5 V Output Range From Single +5 V
–12–
REV. B
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