Electronic Components Datasheet Search 

ADXL05 Datasheet(PDF) 13 Page  Analog Devices 

ADXL05 Datasheet(HTML) 13 Page  Analog Devices 
13 / 20 page ADXL05 –13– REV. B The equivalent rms noise of the bandpass filter is equal to 500 µg / Hz × (1.5 F H )– ( FL /1.5) . For example, the typical rms noise of the ADXL05 using 1 pole ac coupling with a bandwidth of 10 Hz and 1 pole lowpass filter of 100 Hz is: Noise (rms ) = 500 µg / Hz × 1.5(100) – (10 /1.5) = 5,987 µg rms or ≈ 5.9 mg rms Because the ADXL05’s noise is for all practical purposes Gaussian in amplitude distribution, the highest noise amplitudes have the smallest (yet nonzero) probability. Peaktopeak noise is therefore difficult to measure and can only be estimated due to its statistical nature. Table II is useful for estimating the probabilities of exceeding various peak values, given the rms value. Table II. Nominal Peakto % of Time that Noise Will Exceed Peak Value Nominal PeaktoPeak Value 2.0 × rms 32% 4.0 × rms 4.6% 6.0 × rms 0.27% 6.6 × rms 0.1% 8.0 × rms 0.006% RMS and peaktopeak noise (for 0.1% uncertainty) for various bandwidths is estimated in Figure 24. As shown by the figure, device noise drops dramatically as the operating bandwidth is reduced. For example, when operated in a 1 kHz bandwidth, the ADXL05 typically has an rms noise level of 19 mg. With ±5 g applied accelerations, this 19 mg resolution limit is nor mally quite satisfactory; but for smaller acceleration levels the noise is now a much greater percentage of the signal. As shown by the figure, when the device bandwidth is rolled off to 100 Hz, the noise level is reduced to approximately 6 mg, and at 10 Hz it is down to less than 2 mg. Alternatively, the signaltonoise ratio may be improved consid erably by using a microprocessor to perform multiple measure ments and then compute the average signal level. When using this technique, with 100 measurements, the signaltonoise ratio will be increased by a factor of 10 (20 dB). DEVICE BANDWIDTH VS. MEASUREMENT RESOLUTION Although an accelerometer is usually specified according to its full scale (clipping) g level, the limiting resolution of the device, i.e., its minimum discernible input level, is extremely important when measuring low g accelerations. The limiting resolution is predominantly set by the measure ment noise “floor” which includes the ambient background noise and the noise of the ADXL05 itself. The level of the noise floor varies directly with the bandwidth of the measurement. As the measurement bandwidth is reduced, the noise floor drops, improving the signaltonoise ratio of the measurement and in creasing its resolution. The bandwidth of the accelerometer can be easily reduced by adding lowpass or bandpass filtering. Figure 24 shows the typi cal noise vs. bandwidth characteristic of the ADXL05. 3dB BANDWIDTH – Hz 100mg 1mg 10mg 10 1k 100 660mg 66mg 6.6mg Figure 24. Noise Level vs. 3 dB Bandwidth The output noise of the ADXL05 scales with the square root of the measurement bandwidth. With a single pole rolloff, the equivalent rms noise bandwidth is π divided by 2 or approxi mately 1.5 times the 3 dB bandwidth. For example, the typical rms noise of the ADXL05J using a 100 Hz one pole post filter is: Noise (rms ) = 500 µg/ Hz × 100(1.5) = 6,124 µgor 6.1 mg rms For the bandpass filter of Figure 27 where both ac coupling and low pass filtering are used, the low frequency rolloff, FL, is de termined by C4 and R1 and the high frequency rolloff, FH, is determined by the 1pole post filter R3, C5. OBSOLETE 
