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ADXL05JH Arkusz danych(PDF) 14 Page - Analog Devices |
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ADXL05JH Arkusz danych(HTML) 14 Page - Analog Devices |
14 / 20 page ADXL05 REV. B –14– OPTIONAL CAPACITOR FOR 3-POLE FILTERING OUTPUT SCALE FACTOR TRIM 20k Ω R7 71.5k Ω OFFSET AND SCALING AMPLIFIER 2-POLE FILTER BUFFER AMP VOUT VREF 1.8V 8 9 10 PRE-AMP VPR R4b VIN– ADXL05 R6 40.2k Ω 0 g LEVEL TRIM R4a C4 2 4 7 6 AD820 R5 42.2k Ω R1 82.5k Ω R3 82.5k Ω C3 0.01µF +5V R5 3 6 +3.4V 2-POLE FILTER COMPONENT VALUES C4µF 0.0033 0.01 0.033 0.1 3dB BW (Hz) 300 100 30 10 C3µF 0.027 0.082 0.27 0.82 OFFSET AND SCALING AMPLIFIER COMPONENT VALUES FULL SCALE ±1g ±2g ±4g ±5g mV per g 2000 1000 500 400 R4a k Ω 10 10 10 10 GAIN 10.00 4.98 2.50 2.00 R5 k Ω 301 200 100 100 R4b k Ω 24.9 35.7 35.7 45.3 Figure 26. Two-Pole Filtering Circuit with Gain and 0 g Offset Adjustment Bandpass Filtering Figure 27 shows how the combination of ac coupling and low- pass filtering together form a bandpass filter that provides an even greater improvement in noise reduction. BUFFER AMP VOUT 1.8V 8 10 PRE-AMP VPR R1 R3 VIN– ADXL05 C5 C4 VPR R2 9 COMPONENT VALUES ARE APPROXIMATE. 0.002µF 0.002µF 0.0068µF 0.0068µF 0.068µF 1000 200 1000 200 200 30 30 3 1 0.1 49.9 249 49.9 249 249 0.10µF 0.022µF 1.0µF 0.68µF 6.8µF 249 249 249 249 249 640k Ω 640k Ω 640k Ω 640k Ω 640k Ω 300 300 100 100 10 SCALE FACTOR IN mV/ g DESIRED LOW FREQUENCY LIMIT, FL R1 VALUE IN k Ω CLOSEST C4 VALUE DESIRED HIGH FREQUENCY LIMIT, FH R3 IN k Ω CLOSEST C5 VALUE VALUE OF R2 FOR +2.5V 0 g LEVEL Figure 27 AC Coupling and Low-Pass Filtering Used Together to Provide a Bandpass Function Low-Pass Filtering The bandwidth of the accelerometer can easily be reduced by using post filtering. Figure 25 shows how the buffer amplifier can be connected to provide 1-pole post filtering, 0 g offset trim- ming, and output scaling. The table provides practical compo- nent values for various full-scale g levels and approximate circuit bandwidths. For bandwidths other than those listed, use the formula: or simply scale the value of capacitor C4 accordingly, i.e., for an application with a 50 Hz bandwidth, the value of C4 will need to be twice as large as its 100 Hz value. If further noise reduc- tion is needed while maintaining the maximum possible band- width, then a 2- or 3-pole post filter is recommended. These provide a much steeper roll-off of noise above the pole fre- quency. Figure 26 shows a circuit that uses the buffer amplifier to provide 2-pole post filtering. Component values for the 2- pole filter were selected to operate the buffer at unity gain. Ca- pacitors C3 and C4 were chosen to provide 3 dB bandwidths of 10 Hz, 30 Hz, 100 Hz, and 300 Hz. In this configuration, the nominal buffer amplifier output will be +1.8 V ± the 200 mV/g scale factor of the accelerometer. An AD820 external op amp allows noninteractive adjustment of 0 g offset and scale factor. The external op amp offsets and scales the output to provide a +2.5 V ± 2 V output over a wide range of full-scale g levels. BUFFER AMP 0.1µF +5V VOUT 0.022µF C1 C1 +3.4V REF COM 1.8V 8 9 10 1 2 3 4 0.022µF C2 PRE-AMP VPR R1b R3 VIN– 5 ADXL05 6 R2 0 g LEVEL TRIM 50k Ω R1a C4 OPTIONAL SCALE FACTOR TRIM * *TO OMIT THE OPTIONAL SCALE FACTOR TRIM, REPLACE R1a AND R1b WITH A FIXED VALUE 1% METAL FILM RESISTOR. SEE VALUES SPECIFIED IN TABLES BELOW. NOTE: FOR NONINTERACTIVE TRIMS, SET SCALE FACTOR FIRST, THEN OFFSET. COMPONENT VALUES FOR VARIOUS FULL-SCALE RANGES AND BANDWIDTHS FULL SCALE ±1g ±2g ±4g ±5g mV per g 2000 1000 500 400 R1a k Ω 10 10 10 10 R2 k Ω 100 100 100 100 C4 µF 0.056 0.0082 0.0082 0.0056 3dB BW (Hz) 10 100 200 300 R3 k Ω 301 200 100 100 R1b k Ω 24.9 35.7 35.7 45.3 3dB BW = 1 2 π R3 C4 Figure 25. Using the Buffer Amplifier to Provide 1-Pole Post Filtering Plus Scale Factor and 0 g Level Trimming C 4 = 1 (2 π R3)Desired 3dB Bandwidth in Hz |
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