Texas Instruments Xilinx UG230, User Manual

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Spartan-3E FPGA Starter Kit Board User Guide www.xilinx.com 79
UG230 (v1.2) January 20, 2011
Analog to Digital Converter (ADC)
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The amplifier interface is relatively slow, supporting only about a 10 MHz clock frequency.
UCF Location Constraints
Figure 10-5 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 10-3 lists the interface signals between the FPGA and the ADC. The SPI_MOSI,
SPI_MISO, and SPI_SCK signals are shared with other devices on the SPI bus. The
DAC_CS 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 10-6 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 maxim 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 10-5: UCF Location Constraints for the DAC Interface
NET "SPI_MOSI" LOC = "T4" | IOSTANDARD = LVCMOS33 | SLEW = SLOW | DRIVE = 6 ;
NET "AMP_CS" LOC = "N7" | IOSTANDARD = LVCMOS33 | SLEW = SLOW | DRIVE = 6 ;
NET "SPI_SCK" LOC = "U16" | IOSTANDARD = LVCMOS33 | SLEW = SLOW | DRIVE = 8 ;
NET "AMP_SHDN" LOC = "P7" | IOSTANDARD = LVCMOS33 | SLEW = SLOW | DRIVE = 6 ;
NET "AMP_DOUT" LOC = "E18" | IOSTANDARD = LVCMOS33 ;
Table 10-3: ADC Interface Signals
Signal FPGA Pin Direction Description
SPI_SCK U16 FPGA

ADC Clock
AD_CONV P11 FPGA

ADC Active-High shutdown and reset.
SPI_MISO N10 FPGA

ADC Serial data: Master Input, Serial Output. Presents
the digital representation of the sample analog
values as two 14-bit two’s complement binary
values.