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An extended presentation of the FPGA-based measurement system is presented inFig.2. There, all important application specific details concerning the design resourcesare shown.

Fig 2: Detailed block diagram of the system

2. DESCRIPTION OF THE SYSTEM RESOURCES

2.1. Temperature SensorFor this application we choose LM94021 CMOS multi gain temperature sensor from

National Semiconductor [5]. It has an analog output that operates at supply voltage aslow as 1.5V while operating over a wide temperature range from 50C to +150C. Thesensor delivers an output voltage that is inversely proportional to the measuredtemperature. The sensors low voltage supply capability makes it an ideal solution forbattery-powered systems as well as for general sensing applications. The two logicinputs: GS1 and GS0 select the gain of the temperature-to-voltage output transferfunction. Four slopes are selectable: 5.5 mV/C, 8.2 mV/C, 10.9 mV/C, and 13.6mV/C. For our application we use GS1=0 GS0=1 in accordance with the onboard

supply voltage of 3.3V Fig.3.

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Fig 3: LM94021 performance specifications

2.2. Analog-to-digital converter

The analog capture circuit (Fig.4) consists of a Linear Technology low noise, digitallyprogrammable gain pre-amplifier LTC6912-1 that scales the incoming analog signalfrom the sensor output with a gain fixed to -1, followed by LTC1407A-1 analog-to-digitalconverter. Both the pre-amplifier and the ADC are serially controlled by the FPGA.

Fig 4: Detailed view of analog capture circuit

The LTC1407A-1 is a 14-bit, 3Msps ADC with two 1.5Msps simultaneously sampleddifferential inputs, suitable for high speed, portable applications. The analog capturecircuit converts the analog voltage on VINA or VINB and transform it to a 14-bit digital

code D[13:0], as expressed by the eq.1:

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3. MEASUREMENT AND RESULTS

We perform 1-day continued measurement to get the results presented on the chartin Fig.6. There, the results derived with our system is compared with these,simultaneously obtained using a sample Agilent 34401A digital multimeter. The averagecomparative error for both measurement devices is quite low: approx. 0.026[%] with amaximum value of 0.091[%], so we can consider our system very accurate (maximumdiversion -/+1 mV).

Fig 6: Comparison between the data measured with the Agilient multimeter and Picoblaze-based system

After proper reading the analog values, we are able to convert these values tousable temperature reading using two methods: LUT (lookup table) or formula. By usingthe formula these values are computed further to calculate the temperature for thecorresponding points. This is done by modifying sensors manufacturer formula:

(2) ( )112

121 T-T

TT

VVV-V

= ,

where V is in mV and T is in C. T1 and V1 are the coordinates of the lowest

temperature, T2and V2are the coordinates of the highest temperature. In our case wediscuss a temperature range from -50 to +125, and the coordinate from the transfertable for GS=01 is 1955mV for -50C and 521mV for +125C. So we derive thefollowing:

(3)( )

( )( )50--T50-251

19555211955-V

= ( ) ( )50T8.194-1955-V += .

As we need to calculate the temperature from the voltage value, the above formulashould be rewrite as:

(4) 50-

8.194-

1955VT

= or

8.194-

1545.3VT

=

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Fig.7 shows the temperature values over the 1-day measurements calculated usingeq.4.

Fig 7: Temperature vs. corresponding voltage over 1-day measurement trial

4. CONCLUSIONS

The FPGA-based prototype measurement system presented above shows a very

good accuracy, quite simple and cost-effective structure. As it is obvious, the presenteddesign serves as a sample one for a wide range of application aimed to capture variousanalog data. Using PicoBlaze embedded core, we have a very reliable programmabledata processing tool, easy extendable to perform other useful function as calibration,data storage, switching between several sensors, etc. Apart from inclusion otherfunctionality, the basic design described here uses minimum chip resources.

5. REFERENCES

[1] Pong P. Chu ( 2008), FPGA Prototyping by VHDL Examples: Xilinx Spartan-3Version. Wiley-Interscience (February 4, 2008).

[2] Losansky J., Rentzsch M., Guldner, H. Intelligent measurement and controlplatform using Spartan3 FPGA, European Conf.on Power Electronics andAppl.,2005, pp.1-5.

[3] Xilinx UG230 (v1.0). Mar., 2006. Spartan-3E Starter Kit Board User Guide.[4] Xilinx UG129 (v1.1.1). Nov., 2005. PicoBlaze 8-bit Embedded Microcontroller

Users Guide.[5] http://www.national.com/pf/LM/LM94021.html - Multi-Gain Temperature Sensor.