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ADXL50AH Fiches technique(PDF) 10 Page - Analog Devices |
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ADXL50AH Fiches technique(HTML) 10 Page - Analog Devices |
10 / 16 page ADXL50 –10– REV. B calibration are used and there is a desire to eliminate trim po- tentiometers, the design should leave room at either supply rail to account for signal swing and or variations in initial zero g bias. For example, in the circuit in Figure 19, the initial zero g bias of ±250 mV will be reflected to the output by the gain of the R3/R1 network, resulting in an output offset of ±526 mV worst case. The offset, combined with a full-scale signal of 50 g, (+2.0 V) will cause the output buffer amplifier to saturate at the supply rail. The full ±2.25 V output swing of the buffer amplifier can be utilized if the user is able to trim the zero-g bias to exactly 2.5 V. In applications where the full-scale range will be ±25 g or less, a bias trim such as that shown in Figure 20 will almost al- ways be required. VARYING THE OUTPUT SENSITIVITY AND 0 g LEVEL USING THE INTERNAL BUFFER AMPLIFIER The uncommitted buffer amplifier may be used to change the output sensitivity to provide useful full-scale ranges of ±50 g and below. Table II provides recommended resistor values for several standard ranges down to ±10 g. As the full-scale range is decreased, buffer amplifier gain is increased, and the noise con- tribution as a percentage of full scale will also increase. For all ranges, the signal-to-noise ratio can be improved by reducing the circuit bandwidth, either by increasing the demodulator ca- pacitor, C1, or by adding a post filter using the buffer amplifier. Table II. Recommended Resistor Values for Setting the Circuit of Figure 20 to Several Common Full-Scale Ranges Buffer SF in FS (g) Gain mV/g R1 R3 R2 ±50.0 2.11 40 49.9 k 105 k 100 k ±40.0 2.63 50 39.2 k 103 k 100 k ±30.8 3.42 65 40.2 k 137 k 100 k ±26.7 3.95 75 28.7 k 113 k 100 k ±20.0 5.26 100 26.1 k 137 k 100 k ±10.0 10.53 200 23.7 k 249 k 100 k Note that the value of resistor R1 should be selected to limit the output current flowing into VPR to less than 25 µA (to provide a safety margin). For a “J” grade device, this current is equal to: I PR = (2.05 V – The peak full -scale output voltage at V PR )– 1.8V R1 in ohms For a ±50 g full-scale range, R1 needs to be 49.9 kΩ or larger in value; but at the lower full-scale g ranges, if the VPR swing is much less, then it is possible to use much lower resistance val- ues. For this table, the circuit of Figure 20 is used, as a 0 g off- set trim will be required for most applications. In all cases, it is assumed that the zero-g bias level is 2.5 V with an output span of ±2 V. Note that for full scales below ±20 g the self-test is unlikely to operate correctly because the VPR pull-down current is not guar- anteed to be large enough to drive R1 to the required –1.0 V swing. In these cases, the self-test command will cause VOUT to saturate at the rail, and it will be necessary to monitor the self- test at VPR. Self-test can remain operational at VPR for all g ranges listed by keeping R1 > 49.9 k Ω, with the subsequent tradeoff that the required values for R3 will become very large. The user always has the option of adding external gain and fil- tering stages after the ADXL50 to make lower full-scale ranges. Measuring Full-Scale Accelerations Less than 5 g Applications, such as motion detection, and tilt sensing, have signal amplitudes in the 1 g to 2 g range. Although designed for higher full-scale ranges, the ADXL50 may be adapted for use in BUFFER AMP ADXL50 PRE-AMP 0.022 µF C1 C1 0.022 µF C2 COM 0.1 µF +5V V OUT V IN– V PR VREF +3.4V 50k Ω R1 R2 V X +1.8V R3 0g LEVEL TRIM 1.8V 2 3 4 1 5 6 9 8 10 Figure 20. ADXL50 Circuit Using the Buffer Amplifier to Set the Output Scaling and 0 g Offset Level low g applications; the two main design considerations are noise and 0 g offset drift (BH, KH grades recommended). At its full 1 kHz bandwidth, the ADXL50 will typically exhibit 1 g p-p of noise. With ±50 g accelerations this is generally not a problem, but at a ±2 g full-scale level the signal-to-noise ratio will be very poor. However, reducing the bandwidth to 100 Hz or less considerably improves the S/N ratio. Figure 25 shows the relationship between ADXL50 bandwidth and noise. The ADXL50 exhibits offset drifts that are typically 0.02 g per °C but which may be as large as 0.1 g per °C. With the buffer amplifier configured for a 2 g full scale, the ADXL50 will typi- cally drift 1/2 of its full-scale range with a 50 °C increase in temperature. There are several cures for offset drift. If a dc response is not required, for example in motion sensing or vibration measurement applications, consider ac coupling the acceleration signal to re- move the effects of offset drift. See the section on ac coupling. Periodically recalibrating the accelerometer’s 0 g level is another option. Autozero or long term averaging can be used to remove long term drift using a microprocessor or the autozero circuit of Figure 29. Be sure to keep the buffer amplifier’s full-scale out- put range much larger than the measurement range to allow for the 0 g level drift. CALCULATING COMPONENT VALUES FOR SCALE FACTOR AND 0 g SIGNAL LEVEL The ADXL50 buffer’s scale factor is set by –R3/R1 (since the amplifier is in the inverter mode). |
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