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ADXL210JE Folha de dados(PDF) 8 Page - Analog Devices |
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ADXL210JE Folha de dados(HTML) 8 Page - Analog Devices |
8 / 12 page REV. 0 ADXL210E –8– TEMPERATURE – C 1.06 0.94 –45 90 –30 –15 0 1530456075 1.04 1.02 1.00 0.98 0.96 TPC 25. Normalized DCM Period (T2) vs. Temperature nominally 50% duty cycle. The acceleration signal can be deter- mined by measuring the length of the T1 and T2 pulses with a counter/timer or with a polling loop using a low cost micro- controller. An analog output voltage can be obtained either by buffering the signal from the XFILT and YFILT pin, or by passing the duty cycle signal through an RC filter to reconstruct the dc value. The ADXL210E will operate with supply voltages as low as 3.0 V or as high as 5.25 V. A(g) = (T1/T2 – 0.5)/4% 0g = 50% DUTY CYCLE T2(s) = RSET( )/125M T2 T1 Figure 1. Typical Output Duty Cycle APPLICATIONS POWER SUPPLY DECOUPLING For most applications a single 0.1 µF capacitor, CDC, will adequately decouple the accelerometer from signal and noise on the power supply. However, in some cases, especially where digital devices such as microcontrollers share the same power supply, digital noise on the supply may cause interference on the ADXL210E output. This may be observed as a slowly undulating fluctuation of voltage at XFILT and YFILT. If additional decoupling is needed, a 100 Ω (or smaller) resistor or ferrite beads, may be inserted in the supply line of the ADXL210E. CDC FERRITE BEAD 100 RSET YFILT VDD COM ST T2 XOUT YOUT XFILT YFILT ADXL210E XFILT VDD Figure 2. DEFINITIONS T1 Length of the “on” portion of the cycle. T2 Length of the total cycle. Duty Cycle Ratio of the “on” time (T1) of the cycle to the total cycle (T2). Defined as T1/T2 for the ADXL210E/ ADXL210. Pulsewidth Time period of the “on” pulse. Defined as T1 for the ADXL210E/ADXL210. THEORY OF OPERATION The ADXL210E is a complete, dual-axis acceleration measure- ment system on a single monolithic IC. It contains a polysilicon surface-micromachined sensor and signal conditioning circuitry to implement an open loop acceleration measurement architec- ture. For each axis, an output circuit converts the analog signal to a duty cycle modulated (DCM) digital signal that can be decoded with a counter/timer port on a microprocessor. The ADXL210E is capable of measuring both positive and negative accelerations to ±10 g. The accelerometer can measure static acceleration forces such as gravity, allowing it to be used as a tilt sensor. The sensor is a surface micromachined polysilicon structure built on top of the silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is mea- sured using a differential capacitor that consists of independent fixed plates and central plates attached to the moving mass. The fixed plates are driven by 180 ° out of phase square waves. An acceleration will deflect the beam and unbalance the differential capacitor, resulting in an output square wave whose amplitude is proportional to acceleration. Phase sensitive demodulation techniques are then used to rectify the signal and determine the direction of the acceleration. The output of the demodulator drives a duty cycle modulator (DCM) stage through a 32 k Ω resistor. At this point a pin is available on each channel to allow the user to set the signal band- width of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing. After being low-pass filtered, the analog signal is converted to a duty cycle modulated signal by the DCM stage. A single resistor sets the period for a complete cycle (T2), which can be set between 0.5 ms and 10 ms (see TPC 12). A 0 g acceleration produces a |
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