
R
Programmable Pre-Amplifier
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.
The reference design files provide more information on converting the voltage applied on
VINA or VINB to a digital representation (see “Related Resources,” page 77 ).
Programmable Pre-Amplifier
The LTC6912-1 provides two independent, inverting amplifiers with programmable gain.
The purpose of the amplifier is to scale the incoming voltage on VINA or VINB so that it
maximizes the conversion range of the DAC, namely 1.65 ± 1.25V.
Interface
Table 9-1 lists the interface signals between the FPGA and the amplifier. The SPI_MOSI
and SPI_SCK signals are shared with other devices on the SPI bus. The AMP_CS signal is
the active-Low slave select input to the amplifier.
Table 9-1:
AMP Interface Signals
Signal
SPI_MOSI
FPGA Pin
AB14
Direction
FPGA AMP
Description
Serial data: Master Output, Slave Input.
Presents eight-bit programmable gain
settings, as defined in Table 9-2 .
AMP_CS
W6
FPGA AMP
Active-Low chip select. The amplifier gain is
set when the signal returns High.
SPI_SCK
AMP_SHDN
AMP_DOUT
AA20
W15
T7
FPGA AMP
FPGA AMP
FPGA AMP
Clock
Active-High shutdown, reset
Serial data. Echoes previous amplifier gain
settings. Can be ignored in most applications.
Programmable Gain
Each analog channel has an associated programmable gain amplifier (see Figure 9-2 ).
Analog signals presented on the VINA or VINB inputs on the J7 header are amplified
relative to 1.65V. The 1.65V reference is generated using a voltage divider of the 3.3V
voltage supply.
The gain of each amplifier is programmable from -1 to -100, as shown in Table 9-2 .
Table 9-2:
Programmable Gain Settings for Pre-Amplifier
Gain
0
A3
B3
0
A2
B2
0
A1
B1
0
A0
B0
0
Input Voltage Range
Minimum Maximum
-1
-2
0
0
0
0
0
1
1
0
0.4
1.025
2.9
2.275
UG334 (v1.1) June 19, 2008
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