Spartan-3A/3AN FPGA Starter Kit Board User Guide
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75
UG334 (v1.1) June 19, 2008
Analog-to-Digital Converter (ADC)
R
UCF Location Constraints
Figure 9-5
provides the User Constraint File (UCF) constraints for the amplifier interface,
including the I/O pin assignment and I/O standard used.
Analog-to-Digital Converter (ADC)
The LTC1407A-1 provides two ADCs. Both analog inputs are sampled simultaneously
when the AD_CONV signal is applied.
Interface
Table 9-3
lists the interface signals between the FPGA and the ADC. The SPI_SCK signal is
shared with other devices on the SPI bus. The active-High AD_CONV signal is the active-
Low slave select input to the DAC. The DAC_CLR signal is the active-Low, asynchronous
reset input to the DAC.
SPI Control Interface
Figure 9-6
provides an example SPI bus transaction to the ADC.
When the AD_CONV signal goes High, the ADC simultaneously samples both analog
channels. The results of this conversion are not presented until the next time AD_CONV is
asserted, a latency of one sample. The maximum sample rate is approximately 1.5 MHz.
The ADC presents the digital representation of the sampled analog values as a 14-bit, two’s
complement binary value.
Figure 9-5:
UCF Location Constraints for the Pre-amplifier Interface (AMP)
NET
"SPI_MOSI"
LOC
= "AB14"|
IOSTANDARD
= LVCMOS33 |
SLEW
= SLOW |
DRIVE
= 8 ;
NET
"AMP_CS"
LOC
= "W6" |
IOSTANDARD
= LVCMOS33 |
SLEW
= SLOW |
DRIVE
= 8 ;
NET
"SPI_SCK"
LOC
= "AA20"|
IOSTANDARD
= LVCMOS33 |
SLEW
= SLOW |
DRIVE
= 8 ;
NET
"AMP_SHDN"
LOC
= "W15" |
IOSTANDARD
= LVCMOS33 |
SLEW
= SLOW |
DRIVE
= 8 ;
NET
"AMP_DOUT"
LOC
= "T7"
|
IOSTANDARD
= LVCMOS33 ;
Table 9-3:
ADC Interface Signals
Signal
FPGA Pin
Direction
Description
SPI_SCK
AA20
FPGA
Æ
ADC Clock
AD_CONV
Y6
FPGA
Æ
ADC Active-High, initiates conversion process.
ADC_OUT
D16
FPGA
Å
ADC Serial data. Presents the digital representation of the
sample analog values as two 14-bit two’s
complement binary values.
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