Xilinx Virtex-6 FPGA User Manual
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32 www.xilinx.com Virtex-6 FPGA System MonitorUG370 (v1.1) June 14, 2010System Monitor CalibrationCalibration CoefficientsThe calibration coefficients are stored in the status registers at the DRP address locationsshown in Table 18..The offset calibration registers store the offset correction factor for the supply sensor andADC. The offset correction factor is a 10-bit, two’s complement number and is expressed inLSBs. Similar to other status registers, the 10-bit values are MSB justified in the registers.For example, if the supply sensor has an offset of +5 LSBs (approximately5 x 3 mV = 15 mV), then the offset coefficient records –5 LSBs or 3FBh, and Status register08h records 1111 1110 11XX XXXXb.Note: For the ADC offset, 1 LSB is approximately equal to 1 mV.The ADC gain calibration coefficient stores the correction factor for any gain error in theADC. The correction factor is stored in the six LSBs of register 0Ah. These six bits store bothsign and magnitude information for the gain correction factor. If the sixth bit is a logic 1,then the correction factor is positive. If it is 0, then the correction factor is negative.The next five bits store the magnitude of the gain correction factor. Each bit is equivalent to0.1%. For example., if the ADC has a positive gain error of +1% (see Figure 13, page 33),then the gain calibration coefficient records –1% (the –1% correction applied to cancel the+1% error). Since the correction factor is negative, the sixth bit is set to zero. The remainingmagnitude bits record 1%, where 1% = 10 x 0.1% and 10 = 1010 binary. The Statusregister 0Ah records 0000 0000 0000 1010. With five bits assigned to the magnitude,the calibration can correct errors in the range ± 0.1% x (2 5 – 1), or ± 3.1%.Calibration ExampleFigure 13 shows an ADC transfer function containing offset-and-gain errors (red dashedline). The ideal transfer function is shown as a dashed black line. The ADC transferfunction has the form of y = m.x + c (linear). Offset is defined as the ADC output codewhen the input is 0V (where the transfer function crosses the y axis). This offset is removedby digitally subtracting this offset. The result of this offset calibration is shown in Figure 13by the blue dashed line.Table 18: Calibration Coefficient RegistersStatus Register Coefficient Description08h Supply Sensor Offset09h ADC Offset0Ah ADC Gain Errorwww.BDTIC.com/XILINX |
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