76
MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide
www.xilinx.com
UG257 (v1.1) December 5, 2007
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
r
1.25V, centered around the reference voltage, 1.65V.
Hence, 1.25V appears in the denominator to scale the analog input accordingly.
Finally, the ADC presents a 14-bit, two’s complement digital output. A 14-bit, two’s
complement number represents values between -2
13
and 2
13
-1. Therefore, the quantity is
scaled by 8192, or 2
13
.
See
“Programmable Pre-Amplifier”
to control the GAIN settings on the programmable
pre-amplifier.
Figure 10-2:
Detailed View of Analog Capture Circuit
+
–
+
–
+
–
+
–
He
a
der J7
S
PI_MO
S
I
AMP_C
S
S
PI_
S
CK
AMP_
S
HDN
AMP_DOUT
(N10)
(T4)
(U16)
(P7)
(N7)
S
p
a
rt
a
n-
3
E FPGA
LTC 6912-1 AMP
REFAB
REFCD
VINA
VINB
GND
VCC
(
3
.
3
V)
(
3
.
3
V)
(2.5V)
REF = 1.65V
A
B
C
S
/LD
DIN
S
CK
S
HDN
DOUT
S
PI Control Interf
a
ce
A GAIN
B GAIN
S
CK
CONV
S
DO
S
PI Control Interf
a
ce
CHANNEL 1 CHANNEL 0
AD_CONV
(P11)
S
PI_MI
S
O
(E18)
14
14
LTC 1407A-1 ADC
A/D
Ch
a
nnel 0
A/D
Ch
a
nnel 1
3
2
1
0
3
2
1
0
1
3
0
...
1
3
0
...
UG257_10_02_060706
D
13:0
>
@
GAIN
V
IN
1.65
V
–
1.25
V
------------------------------------
u
8192
u
=