Analog Devices AN-377 Application Notes
AN-377
SENSITIVITY – mV/g
0
–180
–360
PHASE SHIFT – Degrees
FREQUENCY – Hz
2000
200
20
10 20k100 1k 10k
0.022µF
0.015µF
0.010µF
0.005µF
a
ONE TECHNOLOGY WAY • P.O. BOX 9106
Increasing the Frequency Response of the ADXL Series Accelerometers
by Mike Shuster, Bob Briano and Charles Kitchin
The ADXL series of accelerometers are specified and
tested for operation over the range from dc to 1 kHz typically. However, in many applications, such as vibration
monitoring, a wider bandwidth is required. The bandwidth of these accelerometers can be increased by decreasing the demodulator capacitance.
The ADXL50 and ADXL05 are closed-loop forced feedback accelerometers (for theory of operation see their
respective data sheets). The sensor acts as a well behaved single-pole system. The demodulator capacitor
sets the dominant pole for the system and thus the –3 dB
bandwidth of the control loop. The –3 dB bandwidth
scales with the inverse of the capacitor value, i.e., halving the capacitor will double the –3 dB bandwidth. The
ADXL50’s sensor beam has a resonance at approximately 24 kHz. The ADXL05’s beam resonance is 12 kHz.
Thus at higher frequencies the single-pole response is
affected by this resonance.
•
NORWOOD, MASSACHUSETTS 02062-9106
APPLICATION NOTE
617/329-4700
•
The frequency response of both accelerometers can be
increased above 10 kHz as long as there is adequate
phase margin to ensure stable operation. However, if an
input pulse or shock signal has energy at the resonant
frequency, this signal will be amplified and cause a
damped ringing response. Figure 1 illustrates the effect
of decreasing the demodulator capacitor for a typical
ADXL50. Figure 2 is the same graph for the ADXL05.
Some temperature and part-to-part variations may
occur. Temperature variation will change the –3 dB point
by typically 4%; low temperature coefficient capacitors
should be used to minimize this effect. Part-to-part
variation can be as much as ±42%. Therefore, a demodulator capacitance value below 0.005 µF is not recommended (to ensure system stability).
190
19
1.9
SENSITIVITY – mV/g
0.19
100 1k
0.022µF
Figure 1. Frequency Response of the ADXL50 for
Various Demodulator Capacitors
PRINTED IN U.S.A. E1892a–5–6/95
0.015µF
0.010µF
FREQUENCY – Hz
0.005µF
10k
0
–90
–180
PHASE SHIFT – Degrees
Figure 2. Frequency Response of the ADXL05 for
Various Demodulator Capacitors
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