BMA145

Data sheet

Bosch Sensortec

3

BMA145 features

3.1

General description

The BMA145 is a fully calibrated, triaxial low-g analog acceleration sensor. It allows

measurements of static as well as dynamic accelerations in all three dimensions. Due to its

three perpendicular axes it gives the absolute orientation in a gravity field and enables free-fall

detection. The sensor is set up as a two-chip arrangement consisting of a three-channel

capacity differential MEMS acceleration-sensing element and an application specific integrated

circuit (ASIC). Both parts are embedded in a standard, surface mountable land grid array

package (LGA). In the following, the LGA packed two-chip arrangement is defined as sensor

module.

The accelerometer features a low-pass characteristic, with a bandwidth limited to 1.5 kHz

(nominal value). This value can further be reduced using external capacitors to improve or

customize noise level. It is advised to reduce bandwidth to minimum value required in the

application.

The bandwidth of all output pins can be selected by the value of the capacitors connected to the

Ax, Ay and Az output. Each channel behaves like an independent low-pass RC filter, given by

the internal 33kΩ resistor and the external capacitor. The resulting RC low pass filter on Ax, Ay

an Az is also valid for the multiplexer output pin. Do not connect an additional external capacitor

on the multiplexer output pin. Refer to chapter 4.8 for details.

3.1.1 MEMS element

The production of the sensing element is based on standard semiconductor- and standard

MEMS processes. The main steps are layer deposition, layer masking and layer structuring in a

technology cycle, similar to the standard semiconductor manufacturing. In detail, the process

cycle starts with the deposition of a thick epitaxial layer on a sacrificial oxide. The large

thickness allows the design of working capacitances of up to 1pF. The poly-layer is patterned by

deep reactive ion etching in an inductively coupled plasma (DRIE-ICP, the so-called Bosch

process). A large aspect ratio and a very high anisotropy is achieved by periodic passivation of

the side walls in between the etch intervals. Afterwards the sacrificial layer is removed. The

sensing element is hermetically sealed by a bulk micromachined cap to prevent damages of the

structure by dicing, packaging and operation of the device.

3.1.2 ASIC

The ASIC is produced in a standard CMOS process. It evaluates, corrects and amplifies the

output signal of the MEMS acceleration-sensing element. Simplified considered, the ASIC

consists mainly out of 3 capacity voltage converters and 3 signal conditioners, a channel

multiplexer, a digital to analog converter an EEPROM memory and an internal RC oscillator. For

customized testing an electrostatic force to the sensor electrodes can be induced and compared

with a nominal condition. Thus, a general statement about the functional capability of the sensor

module can be made at any time.

Rev. 1.3

Page 9 / proprietary information

11 February 2010

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Note: Specifications within this document are subject to change without notice.