Analog Devices AN-396 Application Notes

AN-396

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Understanding Accelerometer Scale Factor and Offset Adjustments

INTRODUCTION

The ADXL50 and ADXL05 accelerometers are small, low
cost, easy to use devices. These modern integrated cir­cuits have an onboard buffer amplifier that allows the
user to change the output scale factor and 0

The output scale factor of an accelerometer is simply
how many volts output are provided per
acceleration. This should not be confused with its reso­lution. The resolution of the device is the lowest
the accelerometer is capable of measuring. Resolution
is principally determined by the device noise and the
measurement bandwidth.

g

•

by Charles Kitchin

g

bias level.

of applied

g

level

NORWOOD, MASSACHUSETTS 02062-9106

APPLICATION NOTE

C2

ADXL50 OR ADXL05

4 1

0.022µF

C1

0.022µF

C1

COM

PRE-AMP

2

3

5

6

8

REF

V

PR

R1

+3.4V

OUTPUT SCALE FACTOR = ——— x VPR OUTPUT
VPR OUTPUT: ADXL50: 19mV/g, ADXL05: 200mV/

617/329-4700

•

1.8V
BUFFER

AMP

10

V

IN–

R3

R3
R1

+5V

C3

0.1µF

9

V

OUT

g

The 0

g

bias level is simply the dc output level of the
accelerometer when it is not in motion or being acted
upon by the earth’s gravity.

SETTING THE ACCELEROMETER’S SCALE FACTOR

Figure 1 shows the basic connections for using the
onboard buffer amplifier to increase the output scale
factor. The nominal output level in volts from V
preamplifier output) is equal to the

g

forces applied to

PR

(the

the sensor (along its sensitive axis) times the output
scale factor of the accelerometer. The ADXL50 has a pre­set scale factor of 19 mV/
the ADXL05’s scale factor is 200 mV/

g

at its preamplifier output, VPR;

g

. The use of the
buffer is always recommended, even if the preset scale
factor is adequate, as the buffer helps prevent any fol­lowing circuitry from loading-down the V

output.

PR

In Figure 1, the output scale factor is simply the output at
V

times the gain of the buffer, which is simply the

PR

value of resistor R3 divided by R1. In all cases, never use
more gain than is needed to provide a convenient scale
factor, as the buffer gain not only amplifies the signal
but any noise or drift as well. Too much gain can
also cause the buffer to saturate and clip the output
waveform.

Figure 1. Basic Buffer Connections

The circuit of Figure 1 is entirely adequate for many ap­plications, but its accuracy is dependent on the
pretrimmed accuracy of the accelerometer, and this will
vary by product type and grade. For the highest pos­sible accuracy, an external trim is recommended. As
shown by Figure 2, this consists of a potentiometer, R1a,
in series with a fixed resistor, R1b.

C2

0.022µF

0.022µF

COM

ADXL50 OR ADXL05

4

C1

2

PRE-AMP

3

C1

5

6

+3.4V

REF

OUTPUT SCALE FACTOR = —————— x VPR OUTPUT
VPR OUTPUT: ADXL50: 19mV/g, ADXL05: 200mV/

8

V

PR

R1a

R1b

R3

(R1a + R1b)

1.8V

10

V

IN–

BUFFER

AMP

g

1

9

R3

0.1µF

V

OUT

+5V

Figure 2. External Scale Factor Trimming

SETTING THE ACCELEROMETER’S 0 g BIAS LEVEL,
AC COUPLED RESPONSE

If a dc (gravity sensing) response is not needed, then the
use of ac coupling between V

and the buffer input is

PR

highly recommended. AC coupling virtually eliminates
any 0

g

drift and allows the maximum buffer gain with­out clipping. The basic ac coupling circuit is shown in
Figure 3. Resistor R1 and Capacitor C4 together form a
high pass filter whose corner frequency is 1/(2 π R1 C4).
This means that this simple filter will reduce the signal
from V

by 3 dB at the corner frequency and it will con-

PR

tinue to reduce it at a rate of 6 dB/octave (20 dB per
decade) for signals below the corner frequency.

The 0

g

offset level of the ADXL50 and ADXL05 acceler­ometers is preset at +1.8 V. There are two simple ways
to change this to a more convenient level, such as +2.5 V
which, being at the middle of the supply voltage, pro­vides the greatest output voltage swing.

When using the ac coupled circuit of Figure 3, only a
single resistor, R2, is required to swing the buffer output
to +2.5 V. Since the “+” input of the buffer is referenced

0.022µF

0.022µF

C2

4 1

C1

2

3

C1

5

COM

+3.4V

REF

ADXL50 OR ADXL05

PRE-AMP

6

8 10

V

PR

C4

R1

1.8V

at +1.8 V, its summing junction, Pin 10, is also held con­stant at +1.8 V. Therefore, to swing the buffer’s output to
the desired +2.5 volt 0

g

bias level, its output must move
up +0.7 V (2.5 V–1.8 V = 0.7 V). Therefore, the current
needed to flow through R3 to cause this change, I
equal to:

Volts

0.7

I

=

R

3

3

in Ohms

R

In order to force this current through R3, the same cur­rent needs to flow from Pin 10 to ground through resis­tor R2. Since Pin 10 is always held at +1.8 V, R2 is equal
to:

Volts

1.8

R

2 =

I

R

3

Therefore, for an ac coupled connection and a +2.5 V 0
output:

V

R2

BUFFER

AMP

IN–

R3

1.8

R

2 =

+5V

C3

0.1µF

V

OUT

2π R1 F

0.7

1

9

BUFFER GAIN = ——

C4 = ————

FOR A +2.5V 0g LEVEL,
IN AN AC COUPLED
CONFIGURATION,
R2 = 2.57 R3

Volts×R

L

Volts

R3

R1

3

= 2.57 ×

R

3

, is

R3

g

ADXL05 RECOMMENDED COMPONENT VALUES

FULL- FACTOR LOW R1 CLOSEST R3 IN kΩ FOR

SCALE IN FREQUENCY IN C4 IN +2.5V 0
RANGE mV/

±2

g

±5

g

±2

g

±5

g

±5

g

SCALE DESIRED R2 VALUE

g

1000 30Hz 49.9 0.10µF 249 640

400 30Hz 127 0.039µF 249 640

1000 3Hz 49.9 1.0µF 249 640

400 1Hz 127 1.5µF 249 640
400 0.1Hz 127 15µF 249 640

LIMIT, F

kΩ VALUE kΩ LEVEL

L

ADXL50 RECOMMENDED COMPONENT VALUES

FULL- FACTOR LOW R1 CLOSEST R3 IN kΩ FOR

SCALE IN FREQUENCY IN C4 IN +2.5V 0
RANGE mV/

±10

g

±20

g

±10

g

±20

g

±10

g

SCALE DESIRED R2 VALUE

g

200 30Hz 24 0.22µF 249 640
100 10Hz 24 0.68µF 127 326
200 3Hz 24 2.2µF 249 640
100 1Hz 24 6.8µF 127 326
200 0.1Hz 24 68µF 249 640

LIMIT, F

kΩ VALUE kΩ LEVEL

L

Figure 3. Typical Component Values for AC Coupled Circuit

–2–

g

g