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ADXL250 Datasheet(PDF) 9 Page - Analog Devices |
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ADXL250 Datasheet(HTML) 9 Page - Analog Devices |
9 / 15 page ADXL150/ADXL250 –9– REV. 0 R3 100k R2 (SEE NOTES) +VS OR GND +VS R1 VOUT +VS 2 SELF-TEST +VS 2 25k 5k ADXL150 GAIN AMP OFFSET NULL COM BUFFER AMP DEMODULATOR SENSOR +VS TP (DO NOT CONNECT) 14 9 10 7 5 CLOCK OP196 3 4 6 2 8 7 NOTES: 0 g “QUICK” CALIBRATION METHOD USING RESISTOR R2 AND A +5V SUPPLY. (a) WITH ACCELEROMETER ORIENTED AWAY FROM EARTH’S GRAVITY (i.e., SIDEWAYS), MEASURE PIN 10 OF THE ADXL150. (b) CALCULATE THE OFFSET VOLTAGE THAT NEEDS TO BE NULLED: VOS =(+2.5V – VPIN10)(R3/R1). 2.5V (R3) VOS ( c) R2 = –––––––– (d) FOR VPIN 10 > +2.5V, R2 CONNECTS TO GND. (e) FOR VPIN 10 < +2.5V, R2 CONNECTS TO +VS. EXT AMP GAIN R1 VALUE FS RANGE DESIRED OUTPUT SCALE FACTOR 76mV/ g 25 g 2.0 49.9k 100mV/ g 20 g 2.6 38.3k 200mV/ g 10 g 5.3 18.7k 400mV/ g 5 g 10.5 9.53k C2 0.1 F C1 0.1 F C4 0.1 F Figure 17. “Quick Zero g Calibration” Connection Adjusting the Zero g Bias Level When a true dc (gravity) response is needed, the output from the accelerometer must be dc coupled to the external amplifier’s input. For high gain applications, a zero g offset trim will also be needed. The external offset trim permits the user to set the zero g offset voltage to exactly +2.5 volts (allowing the maximum output swing from the external amplifier without clipping with a +5 supply). With a dc coupled connection, any difference between the zero g output and +2.5 V will be amplified along with the signal. To obtain the exact zero g output desired or to allow the maximum output voltage swing from the external amplifier, the zero g offset will need to be externally trimmed using the circuit of Figure 20. The external amplifier’s maximum output swing should be limited to ±2 volts, which provides a safety margin of ±0.25 volts before clipping. With a +2.5 volt zero g level, the maxi- mum gain will equal: 2 Volts 38 mV/g Times the Max Applied Acceleration in g The device scale factor and zero g offset levels can be calibrated using the earth’s gravity, as explained in the section “calibrating the ADXL150/ADXL250.” Using the Zero g “Quick-Cal” Method In Figure 18 (accelerometer alone, no external op amp), a trim potentiometer connects directly to the accelerometer’s zero g null pin. The “quick offset calibration” scheme shown in Figure 17 is preferred over using a potentiometer, which could change its setting over time due to vibration. The “quick offset calibra- tion” method requires measuring only the output voltage of the ADXL150/ADXL250 while it is oriented normal to the earth’s gravity. Then, by using the simple equations shown in the figures, the correct resistance value for R2 can be calculated. In Figure 17, an external op amp is used to amplify the signal. A resistor, R2, is connected to the op amp’s summing junction. The other side of R2 connects to either ground or +VS depend- ing on which direction the offset needs to be shifted. OFFSET NULL +VS 200k RIN AT PIN 8 30k 10k SELF-TEST +VS 2 25k 5k ADXL150 GAIN AMP COM BUFFER AMP DEMODULATOR SENSOR +VS TP (DO NOT CONNECT) VOUT 14 8 9 10 7 5 CLOCK C1 0.1 F C2 0.1 F Figure 18. Offset Nulling the ADXL150/ADXL250 Using a Trim Potentiometer |
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