ADXL150/ADXL250

–9–

REV. 0

R3

100k

R2 (SEE NOTES)

R1

2

SELF-TEST

2

25k

5k

ADXL150

GAIN

AMP

OFFSET

NULL

COM

BUFFER

AMP

DEMODULATOR

SENSOR

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:

2.5V (R3)

(

c) R2 = ––––––––

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.

ing on which direction the offset needs to be shifted.

OFFSET

NULL

200k

30k

10k

SELF-TEST

2

25k

5k

ADXL150

GAIN

AMP

COM

BUFFER

AMP

DEMODULATOR

SENSOR

TP

(DO NOT CONNECT)

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