Specifying the dac output voltage, Dac outputs a and b, Dac outputs c and d – Digilent 410-087P-KIT User Manual
Page 70: Dac outputs a and b dac outputs c and d, Figure 9-4
70
Spartan-3E Starter Kit Board User Guide
UG230 (v1.0) March 9, 2006
Chapter 9: Digital to Analog Converter (DAC)
R
The FPGA first sends eight dummy or “don’t care” bits, followed by a 4-bit command. The
most commonly used command with the board is COMMAND[3:0] = “0011”, which
immediately updates the selected DAC output with the specified data value. Following the
command, the FPGA selects one or all the DAC output channels via a 4-bit address field.
Following the address field, the FPGA sends a 12-bit unsigned data value that the DAC
converts to an analog value on the selected output(s). Finally, four additional dummy or
don’t care bits pad the 32-bit command word.
Specifying the DAC Output Voltage
As shown in
, each DAC output level is the analog equivalent of a 12-bit
unsigned digital value, D[11:0], written by the FPGA to the DAC via the SPI interface.
The voltage on a specific output is generally described in
. The reference
voltage, V
REFERENCE
, is different between the four DAC outputs. Channels A and B use a
3.3V reference voltage and Channels C and D use a 2.5V reference. The reference voltages
themselves have a
±5% tolerance, so there will be slight corresponding variances in the
output voltage.
Equation 9-1
DAC Outputs A and B
provides the output voltage equation for DAC outputs A and B. The
reference voltage associated with DAC outputs A and B is 3.3V
± 5%.
Equation 9-2
DAC Outputs C and D
provides the output voltage equation for DAC outputs A and B. The
reference voltage associated with DAC outputs A and B is 2.5V
± 5%.
Equation 9-3
Figure 9-4:
SPI Communications Protocol to LTC2624 DAC
31
0
x x
x x
x x
x x
x x
x x
9 10
a
0
a
1
a
2
a
3
c
0
c
1
c
2
c
3
11
8
7
6
5
4
3
2
1
0
ADDRESS
a
3
a
2
a
1
a
0
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
All
DAC A
DAC B
DAC C
DAC D
1 1 1 1
SPI_MOSI
SPI_SCK
DAC_CS
SPI_MISO
12-bit Unsigned
COMMAND
Don’t Care
UG230_c9_04_021806
msb
lsb
Don’t Care
DATA
Slave: LTC2624 DAC
Master
Spartan-3E
FPGA
V
OUT
D 11:0
[
]
4096
,
---------------------
V
REFERENCE
×
=
V
OUTA
D 11:0
[
]
4096
,
---------------------
3.3V
5%
±
(
)
×
=
V
OUTC
D 11:0
[
]
4096
,
---------------------
2.5V
5%
±
(
)
Ч
=