ADXL150 Analog Devices, Inc., ADXL150 Datasheet - Page 8

• Manufacturer Part Number: ADXL150
• Description: 65 G To 650 G, Low Noise, Low Power , Single/dual Axis Imems? Accelerometers
• Manufacturer: Analog Devices, Inc.

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
consumption. The offset null pin connects to the V
point inside the accelerometer via 30 k , 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.
ADXL150/ADXL250
i
MEM
+V
0.1 F
S
C1
+V
S
0.1 F
S
Accelerometer’s Output
C1
14
Figure 15. Using an External Op Amp to Increase Output Scale Factor
SENSOR
SELF-TEST
14
(DO NOT CONNECT)
Figure 16. AC Coupled Connection Using an External Op Amp
SENSOR
SELF-TEST
ADXL150
(DO NOT CONNECT)
9
ADXL150
TP
9
BUFFER
5
GAIN
CLOCK
2
4
5
TP
5
GAIN
AMP
CLOCK
TYPICAL COMPONENT VALUES FOR AC COUPLED CIRCUIT
COM
GAIN
FS RANGE
AMP
COM
25 g
12.5 g
10 g
DEMODULATOR
7
S
DEMODULATOR
7
/2 reference
OFFSET
1M
332k
249k
0.1 F
R2
OFFSET
NULL
0.1 F
25k
NULL
+V
C2
2
C2
S
25k
+V
8
2
S
5k
0.15 F
0.47 F
0.68 F
+V
OUTPUT SCALE FACTOR = 38mV/ g ––
1Hz
2
C3 VALUE FOR 3dB CORNER FREQ
–8–
8
S
5k
BUFFER
+V
AMP
2
S
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 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).
BUFFER
0.05 F
0.15 F
0.22 F
AMP
3Hz
10
V
OUT
10
C3
0.015 F 0.0075 F
0.047 F 0.022 F
0.022 F 0.01 F
10Hz
R1
R2
+V
2
S
20Hz
3
2
3
2
1M
OP196
R3
R1
OP196
EXTERNAL AMP GAIN = ––––
R3
4
+V
4
7
+V
S
7
0.1 F
S
C4
6
6
C4
0.1 F
V
1M
OUT
R2
OUTPUT
REV. 0