Table of Contents
Advertisement
Chapter 9: Digital to Analog Converter (DAC)
SPI_MISO
SPI_MOSI
DAC_CS
Master
Spartan-3E
SPI_SCK
FPGA
Figure 9-4: SPI Communications Protocol to LTC2624 DAC
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
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
voltage, V
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.
DAC Outputs A and B
Equation 9-2
reference voltage associated with DAC outputs A and B is 3.3V ± 5%.
DAC Outputs C and D
Equation 9-3
reference voltage associated with DAC outputs A and B is 2.5V ± 5%.
70
0
Slave: LTC2624 DAC
x x
x x
0
1
2
3
4
5
6
lsb
Don't Care
12-bit Unsigned
DATA
a
a
3
2
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
1 1 1 1
Figure
9-2, each DAC output level is the analog equivalent of a 12-bit
, is different between the four DAC outputs. Channels A and B use a
REFERENCE
D 11:0
V
-------------------- -
=
OUT
provides the output voltage equation for DAC outputs A and B. The
V
=
OUTA
provides the output voltage equation for DAC outputs A and B. The
V
=
OUTC
www.xilinx.com
a
a
a
a
c
c
7
8
9 10
11
0
1
2
3
0
msb
COMMAND
ADDRESS
a
a
1
0
DAC A
DAC B
DAC C
DAC D
All
[
]
×
V
,
REFERENCE
4096
[
]
D 11:0
×
(
±
)
-------------------- -
3.3V 5%
,
4096
[
]
D 11:0
×
(
±
)
-------------------- -
2.5V 5%
,
4096
Spartan-3E Starter Kit Board User Guide
3 1
x x
x x
x x
x x
c
c
1
2
3
Don't Care
UG230_c9_04_021806
Equation
9-1. The reference
Equation 9-1
Equation 9-2
Equation 9-3
UG230 (v1.0) March 9, 2006
R
Advertisement
Table of Contents
