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ADXL05JH Arkusz danych(PDF) 11 Page - Analog Devices |
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ADXL05JH Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 20 page ADXL05 –11– REV. B BUFFER AMP 0.1µF +5V VOUT 0.022µF C1 C1 +3.4V REF COM 1.8V 6 8 9 10 1 2 3 4 0.022µF C2 PRE-AMP VPR R1b R3 VIN– OUTPUT SCALE FACTOR = x VPR OUTPUT VPR OUTPUT: 200mV/g R3 (R1a + R1b) 5 ADXL05 R1a Figure 21. External Scale Factor Trimming Setting the Accelerometer’s 0 g Bias Level, AC Coupled Response If a dc (gravity) response is not required—for example in motion sensing or vibration measurement applications—ac coupling can be used between the preamplifier output and the buffer input as shown in Figure 22. The use of ac coupling between VPR and the buffer input virtually eliminates any 0 g drift and allows the maximum buffer gain without clipping. Resistor R1 and capacitor C4 together form a high pass filter whose corner frequency is 1/(2 π R1 C4). This means that this simple filter will reduce the signal from VPR by 3 dB at the corner frequency, and it will continue to reduce it at a rate of 6 dB/octave (20 dB per decade) for signals below the corner frequency. Note that capacitor C4 should be a nonpolarized, low leakage type. If a polarized capacitor is used, tantalum types are pre- ferred, rather than electrolytic. With polarized capacitors, VPR should be measured on each device and the positive terminal of the capacitor connected toward either VPR or VIN—whichever is more positive The 0 g offset level of the ADXL05 accelerometer is preset at +1.8 V. This can easily be changed to a more convenient level, such as +2.5 V which, being at the middle of the supply voltage, provides the greatest output voltage swing. When using the ac coupled circuit of Figure 22, only a single re- sistor, R2, is required to swing the buffer output to +2.5 V. Since the “+” input of the buffer is referenced at +1.8 V, its summing junction, Pin 10, is also held constant at +1.8 V. Therefore, to swing the buffer’s output to the desired +2.5 V 0 g bias level, its output must move up +0.7 V (2.5 V – 1.8 V = 0.7 V). Therefore, the current needed to flow through R3 to cause this change, IR3, is equal to: IR 3 = 0.7 Volts R3 in Ohms USING THE INTERNAL BUFFER AMPLIFIER TO VARY THE ACCELEROMETER’S OUTPUT SCALE FACTOR AND 0 g BIAS LEVEL The ADXL05 accelerometer has an onboard buffer amplifier that allows the user to change the output scale factor and 0 g bias level. The output scale factor of an accelerometer is simply how many volts output are provided per g of applied acceleration. This should not be confused with its resolution. The resolution of the device is the lowest g level the accelerometer is capable of mea- suring. Resolution is principally determined by the device noise and the measurement bandwidth. The 0 g bias level is simply the dc output level of the accelerom- eter when it is not in motion or being acted upon by the Earth’s gravity. Setting the Accelerometer’s Scale Factor Figure 20 shows the basic connections for using the onboard buffer amplifier to increase the output scale factor. The nominal output level in volts from VPR (the preamplifier output) is equal to the g forces applied to the sensor (along its sensitive axis) times 200 mV/g. The use of the buffer is always recommended, even if the preset scale factor is adequate, as the buffer helps prevent any following circuitry from loading down the VPR output. BUFFER AMP C3 0.1µF +5V VOUT 0.022µF C1 C1 +3.4V REF COM 1.8V 6 8 9 10 1 2 3 4 0.022µF C2 PRE-AMP VPR R1 R3 VIN– OUTPUT SCALE FACTOR = x VPR OUTPUT VPR OUTPUT: 200mV/g R3 R1 5 ADXL05 Figure 20. Basic Buffer Connections In Figure 20, the output scale factor at Pin 9 (VOUT) is the out- put at VPR times the gain of the buffer, which is simply the value of resistor R3 divided by R1. Choose a convenient scale factor, keeping in mind that the buffer gain not only amplifies the sig- nal but any noise or drift as well. Too much gain can also cause the buffer to saturate and clip the output wave form. The circuit of Figure 20 is entirely adequate for many applica- tions, but its accuracy is dependent on the pretrimmed accuracy of the accelerometer and this will vary by product type and grade. For the highest possible accuracy, an external trim is rec- ommended. As shown by Figure 21, this consists of a potenti- ometer, R1a, in series with a fixed resistor, R1b. |
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Podobny opis - ADXL05JH |
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