74 www.xilinx.com Spartan-3E Starter Kit Board User GuideUG230 (v1.0) March 9, 2006Chapter 10: Analog Capture Circuit RDigital Outputs from Analog InputsThe analog capture circuit converts the analog voltage on VINA or VINB and converts it toa 14-bit digital representation, D[13:0], as expressed by Equation 10-1.Equation 10-1The GAIN is the current setting loaded into the programmable pre-amplifier. The variousallowable settings for GAIN and allowable voltages applied to the VINA and VINB inputsappear in Table 10-2.The reference voltage for the amplifier and the ADC is 1.65V, generated via a voltagedivider shown in Figure 10-2. Consequently, 1.65V is subtracted from the input voltage onVINA 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 CircuitHeader J7SPI_MOSIAMP_CSSPI_SCKAMP_SHDNAMP_DOUT(N10) (T4)(U16)(P7)(N7)Spartan-3E FPGALTC 6912-1 AMPREFABREFCDVINAVINBGNDVCC(3.3V)(3.3V)(2.5V)REF = 1.65VABCS/LDDINSCKSHDNDOUTSPI Control InterfaceA GAIN B GAINSCKCONVSDOSPI Control InterfaceCHANNEL 1 CHANNEL 0AD_CONV(P11)SPI_MISO(E18)1414LTC 1407A-1 ADCA/DChannel 0A/DChannel 132103210 130 ... 130 ...UG230_c10_02_022306D 13:0[ ] GAIN V IN 1.65V–( )1.25V------------------------------------× 8192×=
