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ADXL150JQC 数据表(PDF) 8 Page - Analog Devices |
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ADXL150JQC 数据表(HTML) 8 Page - Analog Devices |
8 / 15 page –8– ADXL150/ADXL250 REV. 0 +VS 2 OUTPUT SCALE FACTOR = 38mV/ g –– R1 R3 SELF-TEST +VS 2 25k 5k ADXL150 GAIN AMP OFFSET NULL COM C1 0.1 F BUFFER AMP DEMODULATOR SENSOR +VS TP (DO NOT CONNECT) 14 9 10 7 5 CLOCK C4 0.1 F +VS OP196 8 3 4 7 6 R1 R3 VOUT 2 C2 0.1 F Figure 15. Using an External Op Amp to Increase Output Scale Factor +VS C4 0.1 F R2 OUTPUT EXTERNAL AMP GAIN = –––– R2 1M +VS 2 VOUT 1M BUFFER GAIN FS RANGE R2 1Hz 3Hz 10Hz 20Hz 2 25 g 1M 0.15 F 0.05 F 0.015 F 0.0075 F 4 12.5 g 332k 0.47 F 0.15 F 0.047 F 0.022 F 5 10 g 249k 0.68 F 0.22 F 0.022 F 0.01 F C3 VALUE FOR 3dB CORNER FREQ TYPICAL COMPONENT VALUES FOR AC COUPLED CIRCUIT C2 0.1 F +VS 2 SELF-TEST +VS 2 25k 5k ADXL150 GAIN AMP OFFSET NULL COM C1 0.1 F BUFFER AMP DEMODULATOR SENSOR +VS TP (DO NOT CONNECT) 14 9 10 7 5 CLOCK OP196 8 3 4 7 6 2 C3 Figure 16. AC Coupled Connection Using an External Op Amp Increasing the iMEMS Accelerometer’s Output Scale Factor Figure 15 shows the basic connections for using an external buffer amplifier to increase the output scale factor. The output multiplied by 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 ampli- fies the signal, but any noise or drift as well. Too much gain can also cause the buffer to saturate and clip the output waveform. Note that the “+” input of the external op amp uses the offset null pin of the ADXL150/ADXL250 as a reference, biasing the op amp at midsupply, saving two resistors and reducing power consumption. The offset null pin connects to the VS/2 reference point inside the accelerometer via 30 k Ω, so it is important not to load this pin with more than a few microamps. It is important to use a single-supply or “rail-to-rail” op amp for the external buffer as it needs to be able to swing close to the supply and ground. The circuit of Figure 15 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 recom- mended. As shown by Figure 20, this consists of a potentiom- eter, R1a, in series with a fixed resistor, R1b. Another option is to select resistor values after measuring the device’s scale factor (see Figure 17). AC Coupling If a dc (gravity) response is not required—for example in vibra- tion measurement applications—ac coupling can be used be- tween the accelerometer’s output and the external op amp’s input as shown in Figure 16. The use of ac coupling virtually eliminates any zero g drift and allows the maximum external amp gain without clipping. Resistor R2 and capacitor C3 together form a high pass filter whose corner frequency is 1/(2 π R2 C3). This filter will reduce the signal from the accelerometer by 3 dB at the corner fre- quency, and it will continue to reduce it at a rate of 6 dB/octave (20 dB per decade) for signals below the corner frequency. Capacitor C3 should be a nonpolarized, low leakage type. If ac coupling is used, the self-test feature must be monitored at the accelerometer’s output rather than at the external amplifier output (since the self-test output is a dc voltage). |
类似零件编号 - ADXL150JQC |
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类似说明 - ADXL150JQC |
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