Spartan-3A/3AN Starter Kit Board User Guide www.xilinx.com 73UG334 (v1.0) May 28, 2007Programmable Pre-AmplifierRFinally, the ADC presents a 14-bit, two’s complement digital output. A 14-bit, two’scomplement number represents values between -2 13 and 2 13 -1. Therefore, the quantity isscaled by 8192, or 2 13.See “Programmable Pre-Amplifier” to control the GAIN settings on the programmablepre-amplifier.The reference design files provide more information on converting the voltage applied onVINA or VINB to a digital representation (see “Related Resources,” page 77).Programmable Pre-AmplifierThe 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 itmaximizes the conversion range of the DAC, namely 1.65 ± 1.25V.InterfaceTable 9-1 lists the interface signals between the FPGA and the amplifier. The SPI_MOSIand SPI_SCK signals are shared with other devices on the SPI bus. The AMP_CS signal isthe active-Low slave select input to the amplifier.Programmable GainEach 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 amplifiedrelative to 1.65V. The 1.65V reference is generated using a voltage divider of the 3.3Vvoltage supply.The gain of each amplifier is programmable from -1 to -100, as shown in Table 9-2.Table 9-1: AMP Interface SignalsSignal FPGA Pin Direction DescriptionSPI_MOSI AB14 FPGAÆAMP Serial data: Master Output, Slave Input.Presents eight-bit programmable gainsettings, as defined in Table 9-2.AMP_CS W6 FPGAÆAMP Active-Low chip select. The amplifier gain isset when the signal returns High.SPI_SCK AA20 FPGAÆAMP ClockAMP_SHDN W15 FPGAÆAMP Active-High shutdown, resetAMP_DOUT T7 FPGAÅAMP Serial data. Echoes previous amplifier gainsettings. Can be ignored in most applications.Table 9-2: Programmable Gain Settings for Pre-AmplifierGain A3 A2 A1 A0 Input Voltage RangeB3 B2 B1 B0 Minimum Maximum0 0 0 0 0-1 0 0 0 1 0.4 2.9-2 0 0 1 0 1.025 2.275
