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AuthorChe-Chang Yang (2010-05-25); recommended: Yeh-Liang Hsu (2010-05-27).

Quick guide to MMA7260QT Tri-axis accelerometer

1.     Key features

The MMA7260QT (Freescale Semiconductor) is a micro-machined, capacitive low-g triaxial accelerometer. It uses a small package (6×6×1.45mm 16-lead QFN) and can be operated with low power voltage (2.2V~3.6V) and low current consumption(500mA; Sleep mode: 3mA). It offers selectable sensing sensitivity of ±1.5g, 2g, 4g, and 6g with integral low-pass filter. These superior features make MMA7260QT applicable to portable electronics, navigation or robotics. Figure 1 show the QFN package and its pin connections are shown in Figure 2. Although a 16-lead package is used, only 8 connections are required, including VDD, VSS (GND), x-, y-, z-axis outputs, Sleep Mode and g-Select1 and 2. The other 8 pins that noted as “N/C” can be left float or unconnected.

Figure 1. MMA 7260QT tri-axis accelerometer in QFN package

 

Figure 2. Pin connections of MMA7260QT

2.     Converting accelerations

Figure 3 shows the axis orientation of the MMA7260QT. The positive signs along x-, y-, and z-axis (with arrows indicated) define the direction that the sensor is accelerated to. The outputs from the MMA7260QT are analog signals with maximal bandwidth response of 350Hz (x- and y-axis) and 150Hz (z-axis). For any axis with no applied acceleration, its output is equal to half the supply voltage (VDD). The output voltage increases from the half VDD level when the sensor is accelerated in the positive direction along its sensitive axis. On the contrary, the signal output is below the half VDD level when the sensor is accelerated in negative direction (or decelerated) along its sensitive axis.

Figure 3. Axis orientation of the MMA7260QT

For a typical VDD=3.3V application, the zero-acceleration output is 0.5×3.3=1.65V. When the sensor is accelerated, the outputs of the sensitive axes deviate from 1.65V and the variation is according to the selected sensitivity S (mV/g, voltage per gravity) as shown in Table 1. For example, if 2g sensitivity is selected, its sensitivity is 600mV/g (g is gravity in the amount of 9.81m/s2) and the voltage within the sensitivity range changes linearly with the measured acceleration (Acc). Equation (1) explains the relationship. If an axis output is 2.35V, the actual measured acceleration along that axis is Acc=. The Equation (1) is valid for x-, y-, and z-axis outputs.

Sensitivity can be selected with 2 logic inputs connected to pin g-Select1 and g-Select2 (Referring to Figure 2). The sensitivity can be changed at anytime during operation. The g-Select pins of the MMA7260QT can be configured with high (1) or low (0) status by microcontroller outputs, as shown in the Table 1. The g-Select pins can be left unconnected for applications only requiring a 1.5g sensitivity.

                                                                         (1)

Table 1. Sensitivity selector

g-Range

Sensitivity (mV/g)

g-Select1

g-Select2

1.5g

800

0

0

2g

600

1

0

4g

300

0

1

6g

200

1

1

The Sleep Mode pin can be connected to a logic inputs for mode switch. Set this pin low to enable MMA7260QT in Sleep Mode that will only consumed trickle current. A high logic input at this pin will switch the sensor to normal operation mode.

3.     Tilt sensing

The MMA7260QT can respond to gravity or constant acceleration due to its capacitive detection principle and mechanism. When gravity is perpendicular to an axis, its axis output is zero-acceleration and therefore is half the VDD (i.e., 1.65V for typical 3.3V application). When gravity is parallel to an axis and the gravity direction is toward the positive direction of that axis, its axis output is half the VDD plus the selected sensitivity (Table 1). For 2g sensitivity application if the sensor is placed horizontally on a surface with top side upward (referring to Figure 1), the x- and y-axis outputs are 1.65V but the z-axis output is 2.25V due to applied gravity. Table 2 further illustrates the MMA7260QT outputs when it is placed at different orientations,

Table 2 The MMA7260QT outputs with respect to different sensor orientations (VDD=3.3V, 2g sensitivity)

Orientation

 

Outputs (Vout)

x-axis

1.65

2.25

1.65

1.05

1.65

1.65

y-axis

2.25

1.65

1.05

1.65

1.65

1.65

z-axis

1.65

1.65

1.65

1.65

2.25

1.05

The gravity response capability of the MMA7260 is useful for accurate tilt sensing with respect to any orthogonal planes. Assume the ,  and  are the tilt angles of x-, y-, and z-axis with respect to horizon, respectively with known accelerations obtained from Equation (1), all the three tilt angles follow sinusoidal relationship that is expressed in Equation (2).

       

                                                                                            (2)

       

Note that Equation (2) stays most accurate when the MMA7260QT is hold static or in the condition without varying acceleration. The resolution (acceleration changed per degree, i.e., the slope the sinusoidal curve) for any axis also varies with tilt angles due to the sinusoidal relationship. Take the x-axis for example, the maximal resolution can be obtained when its tilt  increases from 0° or 180°, and the minimal resolution occurs at  approaches 90° or 270°. Therefore a modified tilt calculation as Equation (3) is suggested and is valid and applicable because it combines other axis outputs and therefore a maximal resolution of tilt sensing can be retained across any rotation and orientation with respect to any axis.

       

                                                                       (3)

       

4.     Typical application schematic

Figure 4 depicts a typical application schematic of the MMA7260QT. The outputs (XOUT, YOUT, ZOUT) can be connected to an analog-to-digital convertor (ADC) for data sampling. Simple RC Low pass filters that use 1kΩ resistor and 0.1mF (104) capacitor equivalent to offering cut-off frequency of 50Hz (-3dB point) can be applied at the sensor output stage. The g-Select and Sleep Mode pin can be connected to digital I/O ports of a microprocessor for logic control. To avoid any noise coupling to the supply power, a 0.1mF bypass capacitor is suggested to be coupled between the VDD and VSS (GND) pins. In PCB layout, this bypass capacitor should be as close as to the sensor for better effect. Physical coupling distance of the accelerometer and the microcontroller (or the length of the analog circuits) should be minimal to ensure lowest noise level. 

Figure 4. Typical application schematic of the MMA7260QT