Digilent 410-087P-KIT - Manual (Page 74)

74 www.xilinx.com Spartan-3E Starter Kit Board User Guide
UG230 (v1.0) March 9, 2006
Chapter 10: Analog Capture Circuit R
Digital Outputs from Analog Inputs
The analog capture circuit converts the analog voltage on VINA or VINB and converts it to
a 14-bit digital representation, D[13:0], as expressed by
Equation 10-1 .
Equation 10-1
The GAIN is the current setting loaded into the programmable pre-amplifier. The various
allowable settings for GAIN and allowable voltages applied to the VINA and VINB inputs
appear in
Table 10-2 .
The reference voltage for the amplifier and the ADC is 1.65V, generated via a voltage
divider shown in
Figure 10-2 . Consequently, 1.65V is subtracted from the input voltage on
VINA or VINB.
The maximum range of the ADC is
±1.25V, centered around the reference voltage, 1.65V.
Hence, 1.25V appears in the denominator to scale the analog input accordingly.
Figure 10-2: Detailed View of Analog Capture Circuit
Header J7
SPI_MOSI
AMP_CS
SPI_SCK
AMP_SHDN
AMP_DOUT
(N10) (T4)
(U16)
(P7)
(N7)
Spartan-3E FPGA
LTC 6912-1 AMP
REFAB
REFCD
VINA
VINB
GND
VCC
(3.3V)
(3.3V)
(2.5V)
REF = 1.65V
A
B
CS/LD
DIN
SCK
SHDN
DOUT
SPI Control Interface
A GAIN B GAIN
SCK
CONV
SDO
SPI Control Interface
CHANNEL 1 CHANNEL 0
AD_CONV
(P11)
SPI_MISO
(E18)
14
14
LTC 1407A-1 ADC
A/D
Channel 0
A/D
Channel 1
3 2 1 0 3 2 1 0 13 0 ... 13 0 ...
UG230_c10_02_022306
D 13:0
[ ]
GAIN
V
IN
1.65V –
( )
1.25V
------------------------------------ Ч
8192
Ч
=