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ADXL202JQC Scheda tecnica(PDF) 8 Page - Analog Devices |
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ADXL202JQC Scheda tecnica(HTML) 8 Page - Analog Devices |
8 / 11 page ADXL202/ADXL210 REV. B –8– Table IV gives typical noise output of the ADXL202/ADXL210 for various CX and CY values. Table IV. Filter Capacitor Selection, CX and CY Peak-to-Peak Noise Estimate 95% Bandwidth CX, CY rms Noise Probability (rms 4) 10 Hz 0.47 µF 1.9 mg 7.6 mg 50 Hz 0.10 µF 4.3 mg 17.2 mg 100 Hz 0.05 µF 6.1 mg 24.4 mg 200 Hz 0.027 µF 8.7 mg 35.8 mg 500 Hz 0.01 µF 13.7 mg 54.8 mg CHOOSING T2 AND COUNTER FREQUENCY: DESIGN TRADE-OFFS The noise level is one determinant of accelerometer resolution. The second relates to the measurement resolution of the counter when decoding the duty cycle output. The ADXL202/ADXL210’s duty cycle converter has a resolu- tion of approximately 14 bits; better resolution than the acceler- ometer itself. The actual resolution of the acceleration signal is, however, limited by the time resolution of the counting devices used to decode the duty cycle. The faster the counter clock, the higher the resolution of the duty cycle and the shorter the T2 period can be for a given resolution. The following table shows some of the trade-offs. It is important to note that this is the resolution due to the microprocessors’s counter. It is probable that the accelerometer’s noise floor may set the lower limit on the resolution, as discussed in the previous section. Table V. Trade-Offs Between Microcontroller Counter Rate, T2 Period and Resolution of Duty Cycle Modulator ADXL202/ Counter- ADXL210 Clock Counts RSET Sample Rate per T2 Counts Resolution T2 (ms) (k ) Rate (MHz) Cycle per g (mg) 1.0 124 1000 2.0 2000 250 4.0 1.0 124 1000 1.0 1000 125 8.0 1.0 124 1000 0.5 500 62.5 16.0 5.0 625 200 2.0 10000 1250 0.8 5.0 625 200 1.0 5000 625 1.6 5.0 625 200 0.5 2500 312.5 3.2 10.0 1250 100 2.0 20000 2500 0.4 10.0 1250 100 1.0 10000 1250 0.8 10.0 1250 100 0.5 5000 625 1.6 initial offset. The easiest way to null this offset is with a calibra- tion factor saved on the microcontroller or by a user calibration for zero g. In the case where the offset is calibrated during manu- facture, there are several options, including external EEPROM and microcontrollers with “one-time programmable” features. DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF The accelerometer bandwidth selected will determine the mea- surement resolution (smallest detectable acceleration). Filtering can be used to lower the noise floor and improve the resolution of the accelerometer. Resolution is dependent on both the ana- log filter bandwidth at XFILT and YFILT and on the speed of the microcontroller counter. The analog output of the ADXL202/ADXL210 has a typical bandwidth of 5 kHz, much higher than the duty cycle stage is capable of converting. The user must filter the signal at this point to limit aliasing errors. To minimize DCM errors the analog bandwidth should be less than 1/10 the DCM frequency. Analog bandwidth may be increased to up to 1/2 the DCM frequency in many applications. This will result in greater dy- namic error generated at the DCM. The analog bandwidth may be further decreased to reduce noise and improve resolution. The ADXL202/ADXL210 noise has the characteristics of white Gaussian noise that contributes equally at all frequencies and is described in terms of µg per root Hz; i.e., the noise is proportional to the square root of the band- width of the accelerometer. It is recommended that the user limit bandwidth to the lowest frequency needed by the application, to maximize the resolution and dynamic range of the accelerometer. With the single pole roll-off characteristic, the typical noise of the ADXL202/ADXL210 is determined by the following equation: Noise rms ()= 500 µg/ Hz × BW ×1.5 At 100 Hz the noise will be: Noise rms ()= 500 µg/ Hz × 100 × (1.5) = 6.12 mg Often the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. Table III is useful for estimating the probabilities of exceeding various peak values, given the rms value. Table III. Estimation of Peak-to-Peak Noise % of Time that Noise Nominal Peak-to-Peak Will Exceed Nominal Value Peak-to-Peak Value 2.0 × rms 32% 4.0 × rms 4.6% 6.0 × rms 0.27% 8.0 × rms 0.006% The peak-to-peak noise value will give the best estimate of the uncertainty in a single measurement. |
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