–8–

ADXL150/ADXL250

REV. 0

2

OUTPUT SCALE FACTOR = 38mV/

g ––

R1

R3

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

C4

0.1 F

OP196

8

3

4

7

6

R1

R3

2

C2

0.1 F

Figure 15. Using an External Op Amp to Increase Output Scale Factor

C4

0.1 F

R2

OUTPUT

EXTERNAL AMP GAIN = ––––

R2

1M

2

1M

BUFFER

GAIN

FS RANGE

R2

1Hz

3Hz

10Hz

20Hz

2

25

g

1M

0.15 F

0.05 F

0.015 F 0.0075 F

4

12.5

g

332k

0.47 F

0.15 F

0.047 F 0.022 F

5

10

g

249k

0.68 F

0.22 F

0.022 F 0.01 F

C3 VALUE FOR 3dB CORNER FREQ

TYPICAL COMPONENT VALUES FOR AC COUPLED CIRCUIT

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

OP196

8

3

4

7

6

2

C3

Figure 16. AC Coupled Connection Using an External Op Amp

Increasing the

Scale Factor

Figure 15 shows the basic connections for using an external

buffer amplifier to increase the output scale factor.

The output multiplied by the gain of the buffer, which is simply

the value of resistor R3 divided by R1. Choose a convenient

scale factor, keeping in mind that the buffer gain not only ampli-

fies the signal, but any noise or drift as well. Too much gain can

also cause the buffer to saturate and clip the output waveform.

Note that the “+” input of the external op amp uses the offset

null pin of the ADXL150/ADXL250 as a reference, biasing the

op amp at midsupply, saving two resistors and reducing power

point inside the accelerometer via 30 k

Ω, so it is important not

to load this pin with more than a few microamps.

It is important to use a single-supply or “rail-to-rail” op amp for

the external buffer as it needs to be able to swing close to the

supply and ground.

The circuit of Figure 15 is entirely adequate for many applica-

tions, 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 recom-

mended. As shown by Figure 20, this consists of a potentiom-

eter, R1a, in series with a fixed resistor, R1b. Another option is

to select resistor values after measuring the device’s scale factor

(see Figure 17).

AC Coupling

If a dc (gravity) response is not required—for example in vibra-

tion measurement applications—ac coupling can be used be-

tween the accelerometer’s output and the external op amp’s

input as shown in Figure 16. The use of ac coupling virtually

eliminates any zero g drift and allows the maximum external

amp gain without clipping.

Resistor R2 and capacitor C3 together form a high pass filter

whose corner frequency is 1/(2

π R2 C3). This filter will reduce

the signal from the accelerometer by 3 dB at the corner fre-

quency, and it will continue to reduce it at a rate of 6 dB/octave

(20 dB per decade) for signals below the corner frequency.

Capacitor C3 should be a nonpolarized, low leakage type.

If ac coupling is used, the self-test feature must be monitored at

the accelerometer’s output rather than at the external amplifier

output (since the self-test output is a dc voltage).