Circuit Note CN-0189 Devices Connected/Referenced High Precision, 1.7 g, Low Power, ADXL203 Dual Axis Accelerometer Circuits from the Lab reference circuits are engineered and 2.7 V to 5.25 V , Micropower, 2-Channel, tested for quick and easy system integration to help solve todays AD7887 125 kSPS, 12-Bit SAR ADC analog, mixed-signal, and RF design challenges. For more Precision, Low Noise, CMOS, Rail-to-Rail information and/or support, visit www.analog.com/CN0189. AD8605 Input/Output, Single Package Op Amp Precision, Low Noise, CMOS, Rail-to-Rail AD8608 Input/Output, Quad Package Op Amp Tilt Measurement Using a Dual Axis Accelerometer will produce a corresponding output voltage on the XOUT or EVALUATION AND DESIGN SUPPORT Y output pins of the device. The X axis and Y axis are OUT Circuit Evaluation Boards perpendicular to one another. The AD8608 quad op amp CN-0189 Circuit Evaluation Board (EVAL-CN0189-SDPZ) buffers, attenuates, and level shifts the ADXL203 outputs so System Demonstration Platform (EVAL-SDP-CB1Z) they are at the proper levels to drive the inputs of the AD7887. Design and Integration Files Schematics, Layout Files, Bill of Materials The rail-to-rail input/output AD8608 is chosen for its low offset voltage (65 V maximum), low bias current (1 pA maximum), CIRCUIT FUNCTION AND BENEFITS low noise (8 nV/Hz), and small footprint (14-lead SOIC or The circuit, shown in Figure 1, incorporates a dual axis TSSOP). ADXL203 accelerometer and the AD7887 12-bit successive The AD7887 is configurable for either dual or single channel approximation (SAR) ADC to create a dual axis tilt operation via the on-chip control register. In this application it measurement system. is configured for dual channel mode, allowing the user to The ADXL203 is a polysilicon surface micromachined sensor monitor both outputs of the ADXL203, thereby providing a and signal conditioning circuit. Acceleration in the X or Y axis more accurate and complete solution. 3.3V 1.36V TO 3.64V 0.63V TO 3.37V 0.2V TO 3.2V 0.1F 10F VCM = 2.5V VCM = 2.0V VCM = 1.7V 5V 5V VDD A1 0.1F 10F AIN0 A3 AD7887 SDP 5V 5V SCLK V 5k 1k S SELF TEST X 10k 1k ST OUT DOUT NC Y OUT A2 COM NC DIN 10F 10F A4 AIN1 NC CS 5V ADXL203 5V 5k 1k GND 10k 1k NOTE: A1, A2, A3, A4 ARE 1/4 AD8608 FOR 2-CHANNEL INPUT, VREF = VDD Figure 1. Dual Axis Tilt Measurement System (Simplified Schematic: Decoupling and All Connections Not Shown) Rev.0 Circuits from the Lab circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2012 Analog Devices, Inc. All rights reserved. 09571-001CN-0189 Circuit Note reduction and antialiasing. The equation for the 3 dB The system maintains an accuracy of 1 over 90 and over bandwidth is: temperature. The circuit provides this precision, performance, and range in a low cost, low power, small footprint, calibration BW = 1/(2 RC ), where R = 32 k (X,Y) dependent solution. The ADXL203 is specified over a minimum With the single poll roll-off characteristic, the typical noise of temperature range of 40C to +105C and is available in an the ADXL203 on a 5 V supply is determined by: 8-terminal ceramic leadless chip carrier package (LCC). RMS Noise = (110 g/Hz) (BW 1.57) CIRCUIT DESCRIPTION Often, the peak-to-peak noise is desired as it gives the best Supply Voltage and Decoupling estimate of the uncertainty in a single measurement peak-to- The ADXL203 requires only one 0.1 F decoupling capacitor as peak noise is estimated by multiplying the rms value by 6. long as there is no noise present at the 140 kHz internal clock Table 2 gives the bandwidth, rms noise, and peak-to-peak noise frequency. If necessary, larger bulk capacitors (1 F to 10 F) or for a given filter capacitor. For this circuit, two 10 F capacitors ferrite beads can be included. create a bandwidth of 0.5 Hz. A minimum capacitance of In order to have output logic levels compatible with the SDP 2000 pF is required in all cases. board, the AD7887 must run on a +3.3 V rail. The rest of the circuit, as indicated in Figure 1, uses the +5 V rail. The Table 1. Filter Capacitor Selection (Cx, Cy) ADXL203 is specified and tested with a nominal supply voltage CX, CY RMS Noise Peak-to-Peak Noise of +5 V. Although the ADXL203 is operational with a supply Bandwidth (Hz) (F) (mg) Estimate (mg) voltage anywhere between 3 V and 6 V, optimum overall 10 0.47 0.4 2.6 performance is achieved at 5 V. Please refer to the ADXL203 50 0.1 1.0 6 data sheet for details regarding performance at other supply 100 0.047 1.4 8.4 voltages. 500 0.01 3.1 18.7 The ADXL203 outputs are ratiometric increasing the supply voltage will act to increase the output voltage. The output Physical Operation of Sensor sensitivity varies proportionally to supply voltage. At VS = 3 V, The sensor is a surface micromachined polysilicon structure the output sensitivity is typically 560 mV/g. At Vs = 5 V, the built on top of the silicon wafer. Polysilicon springs suspend the device has a nominal sensitivity of 1000 mV/g. structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is The zero-g output level is also ratiometric, so the zero-g output measured using a differential capacitor that consists of is nominally equal to VS/2 at all supply voltages. independent fixed plates and plates attached to the moving The output noise of the ADXL203, however, is not ratiometric mass. but absolute in volts. This means the noise density will decrease as the supply voltage increases. This is because the scale factor The fixed plates are driven by 180 out-of-phase square waves. (mV/g) increases while the noise voltage remains the same. Acceleration deflects the beam and unbalances the differential capacitor, resulting in an output square wave whose amplitude For VS = 3 V, the noise density is typically 190 g/Hz and is proportional to acceleration. Phase-sensitive demodulation 110 g/Hz for VS = 5 V. techniques are then used to rectify the signal and determine the Noise, Bandwidth, and Output Capacitor Selection direction of the acceleration. The ADXL203 noise has the characteristics of white Gaussian Input Vector and Part Orientation noise, which contributes equally at all frequencies. It is The input signal to the ADXL203 is not a standard current or described in terms of g/Hz (the noise is proportional to the square root of the accelerometer bandwidth). The user should voltage. Instead, the accelerometer uses the force of gravity as limit bandwidth to the lowest frequency needed by the an input vector to determine the orientation of an object in space. Figure 2 shows the ADXL203 in five different application to maximize the resolution and dynamic range of orientations with respect to the earths surface and the the accelerometer. corresponding output voltages based on the orientation of the The bandwidth is set by a capacitor (CX,Y) on the XOUT and YOUT sensor. pins of the device. These capacitors create a low-pass filter when combined with the internal 32 k output resistor of the When the axis of interest (the X-axis for this example) is ADXL203. These filters are intended primarily for noise oriented parallel to the Earths surface, the sensor experiences a 0 g field, which corresponds to a zero-g bias level of 2.5 V. The output voltage will change according to the sensitivity of the Rev. A Page 2 of 8