ANALOG DEVICES ad AN-377 APPLICATION NOTE ONE TECHNOLOGY WAY P.O. BOX 9106 NORWOOD, MASSACHUSETTS 02062-9106 617/329-4700 Increasing the Frequency Response of the ADXL50 by Mike Shuster and Bob Briano The ADXL50 is specified and tested for operation over the range from dc to 1.3 kHz typically. However, in many applications, such as vibration monitoring, a wider bandwidth is required. The bandwidth of the ADXL50 can be increased by changing the demodulator capacitor. The ADXL50 is a closed-loop forced feedback acceler- ometer (for theary of operation see the data sheet). 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 ioop. The -3 dB bandwidth scales with the inverse of the capacitor value, i.e., halving the capacitor will double the -3 dB bandwidth. The sensor beam has a resonance at approximately 24kHz. Thus at higher frequencies the single-pole response is affected by this resonance. The ADXL50 can be used up to 10 kHz where 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. Some temperature and part-to-part variation may occur. Temperature variation will change the -3 dB point by typically 4%; low temperature coefficient capacitors should be used. Part-to-part variation can be as much as +42%. Therefore, it is not recommended to use a demodulator capacitor value below 0.005 uF to ensure system stability. 190 2 > fw A z | Ao eh /) 5 0.022uF PI 5 1.9 At | 0.015, / Sy FA 0.010nF | a \ 0.005 uF 0.19 ;~ L\ \ 0 ane SSE & g mE AYN -90 ! SN E a a -180 & x Ma 100 1k 10k FREQUENCY - Hz" Figure 1. Frequency Response of the ADXL50 for Various Demodulator Capacitors. PRINTED IN U.S.A. E1892-12-4/94