ADXL150/ADXL250

–11–

REV. 0

2-POLE

FILTER

R2

42.2k

R3

82.5k

R1

82.5k

200k

0

g TRIM

R6

1M

R5

R4

100k

OUTPUT

SCALING

AMPLIFIER

BW

C3

C4

TYPICAL FILTER VALUES

300Hz

0.027 F 0.0033 F

100Hz

0.082 F 0.01 F

30Hz

0.27 F

0.033 F

10Hz

0.82 F

0.1 F

0.1 F

C2

0.1 F

2

SELF-TEST

2

25k

5k

ADXL150

GAIN

AMP

OFFSET

NULL

COM

C1

0.1 F

BUFFER

AMP

DEMODULATOR

SENSOR

TP

(DO NOT CONNECT)

14

9

10

7

5

CLOCK

1/2

OP296

3

1

2

8

8

EXT

AMP

GAIN

R5

VALUE

F.S.

RANGE

DESIRED

OUTPUT

SCALE FACTOR

76mV/

g

±25

g

2.0

200k

100mV/

g

±20

g

2.6

261k

200mV/

g

±10

g

5.3

536k

400mV/

g

±5

g

10.5

1M

C3

C4

2

1/2

OP296

5

4

7

6

Figure 21. Two-Pole Post Filter Circuit

for various full-scale g levels and approximate circuit band-

widths. For bandwidths other than those listed, use the

formula:

Cf

=

1

2

π R3

or simply scale the value of capacitor Cf accordingly; i.e., for an

application with a 50 Hz bandwidth, the value of Cf will need

to be twice as large as its 100 Hz value. If further noise reduc-

tion is needed while maintaining the maximum possible band-

width, a 2- or 3-pole post filter is recommended. These provide

a much steeper roll-off of noise above the pole frequency. Fig-

ure 21 shows a circuit that provides 2-pole post filtering. Com-

ponent values for the 2-pole filter were selected to operate the

first op amp at unity gain. Capacitors C3 and C4 were chosen

to provide 3 dB bandwidths of 10 Hz, 30 Hz, 100 Hz and

300 Hz.

The second op amp offsets and scales the output to provide a

+2.5 V

± 2 V output over a wide range of full-scale g levels.

APPLICATION HINTS

ADXL250 Power Supply Pins

When wiring the ADXL250, be sure to connect BOTH power

supply terminals, Pins 14 and 13.

Ratiometric Operation

Ratiometric operation means that the circuit uses the power

supply as its voltage reference. If the supply voltage varies, the

accelerometer and the other circuit components (such as an

ADC, etc.) track each other and compensate for the change.

Figure 22 shows how both the zero g offset and output sensitiv-

ity of the ADXL150/ADXL250 vary with changes in supply

voltage. If they are to be used with nonratiometric devices, such

as an ADC with a built-in 5 V reference, then both components

should be referenced to the same source, in this case the ADC

reference. Alternatively, the circuit can be powered from an

external +5 volt reference.

POWER SUPPLY VOLTAGE

2.65

2.50

2.35

5.25 5.20 5.15 5.10 5.05 5.00 4.95 4.90

4.85 4.80 4.75

2.60

2.55

2.45

2.40

40.25

38.00

35.75

39.50

38.75

37.25

36.50

Figure 22. Typical Ratiometric Operation

Since any voltage variation is transferred to the accelerometer’s

output, it is important to reduce any power supply noise. Simply

following good engineering practice of bypassing the power supply

right at Pin 14 of the ADXL150/ADXL250 with a 0.1

µF ca-

pacitor should be sufficient.