A DIGITAL OUTPUT ANGULAR ACCELEROMETER
by F. Pasolini
www.DataSheet4UT.choem LIS1R02 is a complete rotational accelerometer system based on a capacitive sensor that uses MEMS
technology, and a set of accompanying electronics that produces a digital output. The device is interfaced to
external hardware using a standard 3-wire serial interface that allows internal registers to be written and rota-
tional acceleration samples to be read.
The MEMS structure consists of a rotor and stator assembly in which capacitive variations occur when the rel-
ative position of the rotor with respect to the stator changes. These capacitive variations are on the order of 50
x 10-18 farads. The MEMS structure also includes actuation electrodes that allow the rotor position to be driven
externally by the processing electronics.
The electronic processing circuitry processes the capacitive variations that occur between the MEMS rotor and
stator. A SigmaDelta architecture is implemented that works to continually restore the rotor to nominal position.
The control effort, or the signal that drives the rotor to nominal, represents the rotational acceleration that is
present at the system location. This control effort is a binary bit stream that is decimated by the electronics to
provide a noise-reduced output
Gain and offset adjustments are applied to the decimated bit stream to produce the acceleration samples. Ac-
celerometer samples then are clocked into a four-deep data FIFO within the IC. The decimation and FIFO stag-
es are clocked in a free-running manner based on the selection of either an internal or external clock source.
1.1 Choosing an External Clock Source
Designers who will use the LIS1R02 to select the clock source which can be either from the CLK_IN pin, from
the internal oscillator or generated by using an embedded PLL.
When the CLK pin is selected as clock source, the designer has the ability to control the rate at which rotational
acceleration samples are generated within the LIS1R02. It takes exactly 224 CLK_IN cycles to generate one
new rotational acceleration sample, therefore the formula for determining the optimal frequency of the CLK_IN
signal is as follows:
where FCLKIN is the frequency of the clock signal that is applied to the CLK_IN pin and Fout si the frequency
at which samples are produced.
If it is possible for the designer to implement a CLK_IN signal that satisfies equation 2.1 perfectly, then the de-
vice will generate one new acceleration sample at the desired rate (1/Ts). In practice, most designers will find it
difficult to supply a clock whose frequency satisfies equation 2.1. Generally, the designer will be restricted to
using a signal for CLK_IN that only approximates equation 2.1. In this case, the acceleration samples will be
generated at a rate that differs from the desired sample rate. The inclusion of the on-chip FIFO data buffer allows
for the proper handling of the accelerometer samples that are produced by the device.
In the case where: