Xilinx Virtex-6 FPGA User Manual

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Xilinx Virtex-6 FPGA User Manual Manual pdf 45 page image

Virtex-6 FPGA System Monitor www.xilinx.com 45UG370 (v1.1) June 14, 2010Application GuidelinesApplication GuidelinesThe Virtex-6 FPGA System Monitor is a precision analog measurement system based on a10-bit Analog-to-Digital Converter (ADC) with an LSB size that is approximately equal to1 mV. To achieve the best possible performance and accuracy with all measurements (bothon-chip and external), a number of dedicated pins for the ADC reference and powersupply are provided. Care must be taken with the connection of these pins to ensure thebest possible performance from the ADC. Basic design guidelines to be considered as partof the requirements for board design are outlined in this section. Finally, an instantiation ofa basic System Monitor in a design is discussed.Reference Inputs (V REFP and V REFN )These high-impedance inputs are used to deliver a differential-reference voltage for theanalog-to-digital conversion process. ADCs are only as accurate as the reference provided.Any reference-voltage error results in a gain error vs. the ideal ADC transfer function(Figure 20, page 43). Errors in the reference voltage affect the accuracy of absolutemeasurements for both on-chip sensors and external channels.For typical usage, the reference voltage between V REFP and V REFN should be maintained at1.25V ± 0.2% using an external reference IC. Reference voltage ICs that deliver 1.25V arewidely available from several vendors. Many vendors offer reference voltage ICs in smallpackages (SOT-23 and SC70). The 1.25V reference should be placed as close as possible andconnected directly to the VREFP input, using the decoupling capacitors recommended inthe reference IC data sheet. The recommended reference connections are illustrated inFigure 24.The System Monitor also has an on-chip reference which is selected by connecting VREFPand V REFN to AGND as shown in Figure 24. Due to reduced accuracy, the on-chipreference does impact the measurement performance of the System Monitor as explainedin this section. The performance with on-chip reference is specified in the Virtex-6 FPGAData Sheet.Analog Power Supply and Ground (AV DD and AV SS)These inputs provide the power supply and ground reference for the analog circuitry (suchas the ADC) in System Monitor.A common mechanism for the coupling of noise into an analog circuit is from the powersupply and ground connections. Excessive noise on the analog supply or ground referenceaffects the ADC measurement accuracy. For example, I/O switching activity can causesignificant disturbance of the digital ground reference plane.Thus, it would not be advisable to use the digital ground as an analog ground reference forSystem Monitor.Similarly, for the digital supplies for the FPGA logic, high switching rates easily result inhigh-frequency voltage variations on the supply, even with decoupling. In an effort tomitigate these effects on the ADC performance, a dedicated supply and ground referenceis provided. Figure 24 illustrates how to use the 2.5V VCCAUX supply to power the analogcircuitry. VCCAUX is filtered using a low-pass network. The filter design depends on theripple and ripple frequency (if any) on the V CCAUX supply. There is also a power-supplyregion specification for the external reference circuit. The filtering should ensure no morethan 2–3 mV of noise on the reference output to minimize any impact on ADC accuracy.www.BDTIC.com/XILINX

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