ADXL50

REV. B

–13–

Figure 24. Circuit Providing Two Pole Post Filtering and 0 g Offset and Scale Factor Trimming

ONE POLE POST FILTERING

Figure 23 shows the ADXL50 buffer amplifier connected to

provide one pole post filtering, 0 g offset trimming, and output

scaling. The table included with the figure lists practical compo-

nent values for various full-scale g levels and approximate circuit

bandwidths. For bandwidths other than those listed, use the

formula:

Capacitor C4 in Farads = 1/(2

π × R3 in Ohms

× 3 dB BW in Hertz)

or simply scale the value of capacitor C4 accordingly; i.e., for a

±20 g application with a 50 Hz bandwidth, the value of C4 will

need to be twice as large as its 100 Hz value or 0.012

µF × 2 =

0.024

µF. The closest standard value of 0.022 µF should then

be used.

TWO POLE POST FILTERING

Figure 24 shows a circuit which uses the ADXL50’s buffer am-

plifier to provide two pole post filtering. An AD820 external op

amp allows noninteractive adjustment of 0 g offset and scale

factor. Component values for the two pole filter were selected to

operate the buffer at unity gain with a Q of one.

Figure 23. Using the Buffer Amplifier to Provide One Pole Post Filtering Plus Scale Factor and 0 g Level Trimming

GAIN

10.53

5.26

2.63

2.11

FULL

SCALE

±10 g

±20 g

±40 g

±50 g

mV

per g

200

100

50

40

R4a

k

Ω

5

5

10

10

R4b

k

Ω

21.5

23.7

34

45.3

R5

k

Ω

249

137

105

105

OFFSET & SCALING AMPLIFIER

COMPONENT VALUES

3dB

BW(Hz)

300

100

30

10

C3

µF

0.027

0.082

0.27

0.82

C4

µF

0.0033

0.01

0.033

0.1

2 POLE FILTER

COMPONENT VALUES

BUFFER

AMP

ADXL50

PRE-AMP

0.022

µF

C1

C1

0.022

µF

C2

COM

0.1

µF

+5V

VREF

R3

1.8V

C4

R5

42.2k

Ω

82.5k

Ω

R1

82.5k

Ω

R4a

R4b

SCALE

FACTOR

TRIM

2

6

+5V

0.01

µF

7

3

4

AD820

R5

OUTPUT

OPTIONAL CAPACITOR

FOR 3 POLE FILTERING

20k

Ω

40.2k

Ω

71.5k

Ω

R6

R7

0g

LEVEL

TRIM

10

6

8

9

1

4

2

3

5

C3

2 POLE FILTER

+3.4V

OFFSET AND

SCALING

AMPLIFIER

3dB BW =

1

2

RECOMMENDED COMPONENT VALUES FOR VARIOUS

FULL SCALE RANGES AND A 300Hz BANDWIDTH

FULL

SCALE

±10 g

±20 g

±40 g

±50 g

mV

per g

200

100

50

40

3dB

BW (Hz)

300

300

300

300

R1a

k

Ω

5

5

10

10

R1b

k

Ω

21.5

23.7

34

45.3

R3

k

Ω

249

137

105

105

R2

k

Ω

100

100

100

100

C4

µF

0.0022

0.0039

0.0056

0.0056

REPLACE R1a AND R1b WITH A FIXED VALUE 1%

METAL FILM RESISTOR. SEE VALUES SPECIFIED

IN TABLE II.

BUFFER

AMP

ADXL50

PRE-AMP

0.022

µF

C1

C1

0.022

µF

C2

COM

0.1

µF

+5V

REF

+3.4V

50k

Ω

R1b

R2

R3

0g

LEVEL

TRIM

1.8V

C4

R1a

OPTIONAL SCALE

FACTOR TRIM

*

2

3

4

1

5

6

9

8

10