Specifying The Dac Output Voltage; Dac Outputs A And B - Xilinx MIcroBlaze Development Spartan-3E 1600E Kit User Manual

Chapter 9: Digital to Analog Converter (DAC)
master. The response from the DAC can be ignored although it is a useful to confirm
correct communication.
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%.
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SPI_MISO
0
SPI_MOSI
x x x x 0 1 2
DAC_CS lsb
SPI_SCK Don't Care
Figure 9-4: SPI Communications Protocol to LTC2624 DAC
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
4096
provides the output voltage equation for DAC outputs A and B. The
D 11:0
-------------------- -
V =
OUTA
MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide
Slave: LTC2624 DAC
3 4 5 6 7 8 9 10 11 a 0 a 1 a 2
msb
12-bit Unsigned
DATA
a a a a
ADDRESS
0 1 2 3
DAC A
0 0 0 0
0 0 0 1 DAC B
DAC C
0 0 1 0
DAC D
0 0 1 1
All
1 1 1 1
Equation
V
REFERENCE
3.3V 5%
4096
a 3 c 0 c 1 c 2 c 3 x x x x x x x x
Don't Care
COMMAND
UG257_09_04_060606
9-1. The reference
Equation 9-1
Equation 9-2
UG257 (v1.1) December 5, 2007
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