MOTOROLA Freescale Semiconductor, Inc. SEMICONDUCTOR APPLICATION NOTE ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 Order this document by AN4004/D AN4004 ±2g Acceleration Sensing Module Based on a ±40g Integrated Accelerometer INTRODUCTION ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 Freescale Semiconductor, Inc... by: Arnaud Delpoux Application Engineer, Toulouse, France and Brandon Loggins Application Engineer, Phoenix, USA Micromachined accelerometers, with built–in signal conditioning and calibration are widely used in automotive safety devices such as airbag modules. Automotive applications for accelerometers also include comfort features such as active suspension. For such an application, a ±2g accelerometer is required. But most accelerometers offered today that are able to provide sufficient sensitivity are expensive and/or unavailable in production volumes. With the circuitry described herein, the accelerometer can be used for sensing acceleration in the range of ±2g, with performance in line with the technical requirements of this application. Figure 1. 40G–2G ±2g Sensing Module Evaluation Board REV 5 Motorola Sensor Device Data Motorola, Inc. 1999 For More Information On This Product, Go to: www.freescale.com 1 Freescale Semiconductor, Inc. AN4004 ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 EVALUATION BOARD DESCRIPTION The information required to use evaluation board “40G–2G” follows and a discussion of the design appears in the Design Considerations section. Electrical Characteristics The following electrical characteristics are included as a guide to operation. Designator Qty. Description Value/Part Number R1 1 1/4 Watt Resistor 90.9 K R2, R8 2 1/4 Watt Resistor 162 K R5, R6, R7, R9, R10 5 1/4 Watt Resistor 11 K R3, R4 2 1/4 Watt Resistor 432 K C1–C6 6 Ceramic Capacitor 0.1 mF U1 1 Quad Rail–to–Rail Op Amp MC33204P U2 1 40g Accelerometer MMA2200W ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 Table 2. Parts List Characteristics Symbol Min Typ Max Units Supply Voltage VCC 4.75 5.0 5.25 Volts Supply Current IO — 7.0 — mA Acceleration Range G –2.0 — +2.0 g DESIGN CONSIDERATIONS 25 Using a 40g accelerometer to measure ±2g of acceleration can be tricky. Motorola’s accelerometer provides a wide bandwidth of 400 Hz for acceleration sensing. In many low g applications, such as active suspension, the bandwidth requirements are considerably lower. Limiting the bandwidth of the accelerometer can reduce noise. After reducing the bandwidth, gain can be applied to provide higher sensitivity for low g acceleration measurements. The design challenge is how to bandpass filter the accelerometer’s output with gain using a few low cost components. To accommodate the accelerometer’s wide dynamic range, since the supply voltage is limited to 5 volts, it is necessary to use rail to rail operational amplifiers, such as Motorola’s MC33204P. In this design, the output signal passes through a biquad filter stage, then through an additional low–pass gain stage to provide ±2g sensing capability. The biquad filter, shown in Figure 3, with some gain, is a good choice for bandpass filtering the accelerometer output when a high quotient factor, Q, is desired. The gain is set by Rg and Rb with the high cutoff frequency being set by Rb and C1. The low cutoff frequency is set by Rf and C2. 20 C2 Zero G Output Voff 2.0 2.1 2.2 Volts ∆V/∆G 850 1000 1150 mV/g Low Cutoff Frequency — 0.8 0.9 1.0 Hz High Cutoff Frequency — 4.0 5.0 6.0 Hz Operating Temperature Ta –40 — +85 °C Sensitivity Table 1. Electrical Characteristics Frequency Response Below is the simulated typical output frequency response using the devices listed in the parts list below. The system provides a gain of 25 over a 5 Hz bandwidth. 30 Av Freescale Semiconductor, Inc... Function The evaluation board shown in Figure 1, when supplied with an accelerometer, provides a ±2g full scale acceleration measurement. The output is an analog signal. It nominally supplies 2.1 volts at zero g and has a sensitivity of 1000 mV/g. It is easily interfaced with a microcontroller’s A/D input. A through–hole area is provided on the PCB for the designer to add other circuitry as needed. Evaluation Board Content Board contents are described in the parts list shown in Table 2. A schematic and silk screen plot are shown in Figures 4 and 5. 15 Rf 10 C1 5.0 Rb Rg Vin 0 0.1 1.0 10 Hz 100 1000 – + Rf – + R R – + BANDPASS Figure 2. ±2g Biquad Filter with Additional Gain Stage Frequency Response Figure 3. Biquad Active Filter 2 For More Information On This Product, Go to: www.freescale.com Motorola Sensor Device Data Freescale Semiconductor, Inc. ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 AN4004 APPLICATION Integrating the accelerometer with the biquad amplifier is quite simple. First, in order to minimize the noise level, the accelerometer is properly bypassed, as indicated in the data sheet, with one 0.1 mF ceramic capacitor between VCC (pin 8) and Ground (pin 7). The self–test feature remains unused in this design. Pin 11 is tied to VCC, as indicated in the data sheet. The output signal is taken on pin 5. The remaining pins of the accelerometer are unused. C4 C5 U2 R4 R3 ACCELEROMETER 4 8 ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 Freescale Semiconductor, Inc... 7 U1 C3 ST R1 GND OUT 13 – + 12 VOFF C6 R2 5 R5 VS VCC VCC R10 C1 14 R8 4 2 – 3 + R6 U1 1 R9 VOFF VOFF 6 – 5 + R7 7 9 10 U1 – + 8 VOUT U1 VOFF Figure 4. ±2g Acceleration Sensing Module EVB Schematic In this design, the biquad filter stage gain is set to 1.76 with R2 (160 kW) and R1 (91 kW). A higher gain cannot be set for this stage, otherwise some operational amplifiers saturate when the accelerometer output drifts over temperature. The upper cutoff frequency is set with R2 (160 kW) and C3 (0.1 mF). Two capacitors, C4 (0.1 mF) and C5 (0.1 mF), are needed to set the lower cutoff frequency. A final low pass filter stage, with a gain of 14.55, brings the overall gain to 25. Resistors R8 (160 kW) and R7 (11 kW) set the gain. The amplifier is low passed with C6 (0.1 mF) to remove any high frequency noise in the signal. The output can then be connected to a microcontroller’s A/D converter by a simple direct connection from the evaluation board analog Motorola Sensor Device Data output VOUT to the A/D input. Using the MC68HC11 as an example, the output is connected to any of the E ports. Since the accelerometer signal passes through 3 inverter stages, the positive direction of acceleration is reversed. CONCLUSION Perhaps the most noteworthy aspect to the ±2g sensing module described here is the ease with which it can be designed. Only two dual, or one quad, operational amplifier and a few resistors and capacitors are required. The result is a simple and inexpensive circuit that is capable to measure acceleration within the range of ±2g with an analog output that can be directly interfaced to a microcontroller. For More Information On This Product, Go to: www.freescale.com 3 Freescale Semiconductor, Inc. AN4004 ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 40G–2G U2 MMAS40G10D XL C2 C1 C5 C4 R3 +5 V + U1 R1 OUT VOUT – R2 – GND C3 + C7 GND R5 R4 R9 R7 R10 R8 MOTOROLA ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005 Freescale Semiconductor, Inc... R6 C6 08/96 Figure 5. Silk Screen Mfax is a trademark of Motorola, Inc. Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motorola data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. 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