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ADXL105_15 Datasheet(Arkusz danych) 7 Page  Analog Devices 

7 page REV. A –7– ADXL105 Output Scaling The acceleration output (AOUT) of the ADXL105 is nominally 250 mV/g. This scale factor may not be appropriate for all appli cations. The UCA may be used to increase the scale factor. The simplest implementation would be as shown in Figure 14a. Since the 0 g offset of the ADXL105 is 2.5 V ± 625 mV, using a gain of greater than 4 could result in having the UCA output at 0 V or 5 V at 0 g. The solution is to add R3 and VR1, as shown in Figure 14b, turning the UCA into a summing amplifier. VR1 is adjusted such that the UCA output is V DD/2 at 0 g. C R1 OUT R2 IN f–3dB = 1 2 CR1 GAIN = – R1 R2 VMID a. 1Pole LowPass Filter 0.22 F OUT 20k IN f–3dB = 30Hz 20k 0.18 F VMID b. 2Pole Bessel LowPass Filter R1 OUT R2 IN f–3dB = 1 2 CR2 GAIN = – R1 R2 C R3 R3 2.5 R1 ~ ~ VMID VMID c. 1Pole HighPass Filter 44.2k OUT IN f–3dB = 10Hz 59k 0.39 F 0.39 F VMID d. 2Pole Bessel HighPass Filter Figure 15. UCA Used as Active Filters* Device Bandwidth vs. Resolution In general the bandwidth selected will determine the noise floor and hence, the measurement resolution (smallest detectable acceleration) of the ADXL105. Since the noise of the ADXL105 has the characteristic of white Gaussian noise that contributes equally at all frequencies, the noise amplitude may be reduced by simply reducing the bandwidth. So the typical noise of the ADXL105 is: Noise (rms) = (225 µg/√Hz) × (√Bandwidth × K) Where K ≈ 1.6 for a singlepole filter K ≈ 1.4 for a 2pole filter So given a bandwidth of 1000 Hz, the typical rms noise floor of an ADLX105 will be: Noise = (225 µg/√Hz) × (√1000 × 1.6) = 9 mg rms for a singlepole filter and Noise = (225 µg/√Hz) × (√1000 × 1.4) = 8.4 mg rms for 2pole filter Often the peak value of the noise is desired. Peaktopeak noise can only be estimated by statistical means. Table I may be used for estimating the probabilities of exceeding various peak values given the rms value. The peaktopeak noise value will give the best estimate of the uncertainty in a single measurement. Table I. Estimation of PeaktoPeak Noise Nominal Peakto % of Time that Noise Will Peak Value Exceed PeaktoPeak Value 2 × rms 32% 3 × rms 13% 4 × rms 4.6% 5 × rms 1.2% 6 × rms 0.27% 7 × rms 0.047% 8 × rms 0.0063% The UCA may be configured to act as an active filter with gain and 0 g offset control as shown in Figure 16. 0.1 F OUT IN GAIN = 2 f–3dB = 30Hz 0.1 F 100k 47k 47k 10k VDD 47k Figure 16. UCA Configured as an Active LowPass Filter with Gain and Offset EMC and Electrical Noise The design of the ADXL105 is such that EMI or magnetic fields do not normally affect it. Since the ADXL105 is ratiomet ric, conducted electrical noise on VDD does affect the output. This is particularly true for noise at the ADXL105’s internal clock frequency (200 kHz) and its odd harmonics. So maintain ing a clean supply voltage is key in preserving the low noise and high resolution properties of the ADXL105. One way to ensure that VDD contains no high frequency noise is to add an RC lowpass filter near the VDD pin as shown in Figure 17. Using the component values shown in Figure 17, noise at 200 kHz is attenuated by approximately –23 dB. As suming the ADXL105 consumes 2 mA, there will be a 100 mV drop across R1. This can be neglected simply by using the ADXL105’s VDD as the AtoD converter’s reference voltage as shown in Figure 17. *For other corner frequencies, consult an active filter handbook. 
