Embedding Temperature Information ADXL202's Outputs Harvey Weinbe

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Embedding Temperature Information ADXL202's Outputs Harvey Weinbe

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AN-600 APPLICATION NOTE
Embedding Temperature Information ADXL202's Outputs
Harvey Weinberg
INTRODUCTION Many cases exist where it's important know temperature ADXL202. Sometimes temperature information necessary control data logging requirements data loggers used monitor articles handled shipping) high precision tilt sensor applications (for temperature compensation zero drift accelerometer). Since converter required when using ADXL202's duty cycle outputs, preferable have some means acquiring temperature information digitally. This application note outlines simple method embedding temperature information from TMP36 voltage output temperature sensor duty cycle acceleration output ADXL202. converter required, additional necessary. BASIC PRINCIPLE OPERATION duty cycle output period proportional current flowing through RSET. voltage typically 1.25 recommended range RSET resistor (125 1.25 current Normally RSET connected between ground, noise free voltage source between 1.25 acceptable. Figure TMP36 voltage output temperature sensor connected RSET. TMP36 output voltage
RSET 127k OUTPUT CASES SHOWN PERIOD 1355
PERIOD 1855
PERIOD 2915
XOUT YOUT
TMP36
RSET
MICROCONTROLLER
ADXL202
RSET YFILT XFILT
Figure Using Temperature Sensor Embed Temperature Information into Acceleration Duty Cycle Output ADXL202
most applications, period need measured every cycle (this discussed thoroughly "Using ADXL202 Duty Cycle Output" application note). Since temperature change normally speed phenomenon, this will affect accuracy temperature measurements. Measuring period times second should more than sufficient. subroutine used determine temperature from period must added user's firmware. Since change completely linear over temperature, subroutine's complexity will vary proportion temperature accuracy required. Table shows typical period versus temperature. Using simple calculation
0.75 (0.01 Volts
where temperature Over range -40°C +70°C, output resulting current
RSET
1.25 1.25 RSET RSET RSET
Temperature
1735
Using resistor RSET, current over -40°C +70°C temperature range.
where Temperature This technique will result temperature readings that accurate ±5.5°C over range -20°C +40°C. more accurate
REV.
Analog Devices, Inc., 2002
AN-600
temperature readings required, simple look-up table used. Alternatively, both initial constant (1735) divisor (26) modified higher accuracy over narrower temperature range. Table Temperature Circuit Figure Temp. (°C) 1250 1300 1355 1420 1490 1565 1650 1740 1855 1980 2110 2280 2450 2660 2915 3230 3620 Calculated Temp. -18.7 -16.7 -14.5 -12.1 -9.4 -6.5 -3.3 +0.2 +4.5 +9.5 +14.4 +27.5 +35.6 +45.5 +57.5 +72.5 example, consider shipping conditions recorder item that must kept 10°C 1°C. From Table that 10°C produces period 1855 Assuming that interested high accuracy over 15°C range, that period changes from 1980 15°C 1740 5°C. Therefore assume that change
1980 1740 15°C
then modify temperature equation
Temperature
1855
which will accurate ±0.2°C over 15°C range.
REV.
PRINTED U.S.A.
E03045-0-7/02(0)

Embedding Temperature Information ADXL202's Outputs Harvey Weinbe

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