ASACK0003

AN 003
Multiplexing Tri-Axis Accelerometer Outputs
Introduction
A Kionix tri-axis accelerometer with analog outputs provides three output voltages
(Xout, Yout, Zout) which are proportional to the respective accelerations in those
directions. However, with three analog outputs to digitize, it is possible that the
system microprocessor does not have the necessary A-D converters. One solution is
to use the internal multiplexing capability of several Kionix accelerometer products to
multiplex the three outputs to one analog signal. Another solution is to use an off
the shelf multiplexer to multiplex the three outputs of the tri-axis accelerometer to
one analog signal.
By multiplexing, only one A-D channel is required and the system will maintain the
performance of the analog output at a high data sampling rate. Even when
multiplexing all three accelerometer outputs, the maximum data sampling rate is still
gated by the speed of the system microprocessor’s A-D converter. This application
note recommends two Texas Instruments multiplexers, CD74HCT4051 and
SN74CBTLV3253, and provides technical information on how to use the internal
multiplexer of several Kionix products.
Using the Internal Multiplexer
The KXPA4 features an integrated 3-channel multiplexer. The KXPB5, KXP94, and
KXR94 feature an integrated 4-channel multiplexer. This feature reduces system
MCU requirements to only 1 ADC and 2 digital I/O’s.
Multiplexer Data Select
The KXPA4, KXPB5, KXP94, and KXR94 use two select (S0, S1) inputs to control the
data flow from Vmux. When a microprocessor toggles the select inputs, the desired
output is attained based on the select table. Note that logic 0 is GND and logic 1 is
Vdd.
36 Thornwood Dr. – Ithaca, NY 14850
tel: 607-257-1080 – fax: 607-257-1146
www.kionix.com - [email protected]
S1
0
0
1
1
KXPA4 Output Select
S0
Vmux
0
X Output
1
Z Output
0
Y Output
1
Y Output
S1
0
0
1
1
KXPB5 Output Select
S0
Vmux
0
X Output
1
Y Output
0
Z Output
1
Aux. In
© Year Kionix, Inc.
Day Month Year
Page 1 of 6
AN 003
S1
0
0
1
1
S1
0
0
1
1
KXP94 Output Select
S0
Vmux
0
X Output
1
Z Output
0
Y Output
1
Aux. In
KXR94 Output Select
S0
Vmux
0
X Output
1
Z Output
0
Y Output
1
Aux. In
The following schematic shows a KXPB5 (U1) connected to a generic microcontroller
(U2).
Fig. 1) KXPB5 connection to a microcontroller
Two digital I/O lines (DIO1 and DIO2) are connected to S0 and S1 of the KXPB5 to
control the switching of the multiplexer. The analog output of the KXPB5 (Vmux) is
connected to one analog to digital converter (ADC) input of the microcontroller. The
remaining connections to ground or Vdd are all that are needed to make the KXPB5
function.
Data Sampling Rate
When operating in their multiplexed mode, these parts have the ability to achieve
very high data sampling rates. Internally, the sensor elements (X, Y, and Z) are
sequentially sampled in a “round robin” fashion at a rate of 32kHz per axis. Note that
© Kionix 2007
Date
Page 2 of 6
AN 003
this is a differential capacitance sampling of each sensor element, which stores an
analog voltage on the filter cap for each axis. Combine this high sensor element
sampling rate with the short 5µS settling time of the integrated multiplexer, and the
user can achieve a performance very close to that of the 3 separate analog outputs.
This is more than sufficient to eliminate any aliasing in the final application since the
Kionix accelerometer will be operating with a typical bandwidth of ~50Hz and a
maximum of 2500Hz.
Using an External Multiplexer
The KXPC4 is a Kionix tri-axis accelerometer with analog outputs which does not
have an internal multiplexer. If multiplexing of the outputs is desired for the
application, then an external multiplexer can be used. Kionix recommends using one
of two Texas Instruments multiplexers, the CD74HCT4051 or SN74CBTLV3253. Of
course, you are free to use whichever multiplexer you are most familiar with.
TI CD74HCT4051
The TI CD74HCT4051 is a 1-of-8 channel high-speed CMOS logic analog multiplexer
in a 16-pin SOIC package. It controls analog voltages that may vary across the
voltage supply range from VCC to VEE. Please refer to the TI CD74HCT4051 website
at http://focus.ti.com/docs/prod/folders/print/cd74hct4051.html for product
summaries, power requirements, specifications, and schematics.
When multiplexing the KXPC4, only three of the eight input channels are needed.
Data flow of a specified output on pin Vmux can be attained by appropriately
toggling the select pins (S1, S0) with the system microprocessor. The schematic in
Fig. 2 shows the recommended connections to the CD74HCT4051.
© Kionix 2007
Date
Page 3 of 6
AN 003
Fig. 2) KXPC4/CD74HCT4051 Schematic
As previously stated, the system microprocessor will need to toggle the multiplexer
select pins (S0, S1) to obtain the desired accelerometer output. The CD74HCT4051
output select table below shows the select pin inputs required to obtain a specified
output.
CD74HCT4051 Output Select
S0
S1
Vmux
0
0
1
0
1
0
Output X
Output Y
Output Z
CD74HCT4051 Output Select Table
TI SN74CBTLV3253
The TI SN74CBTLV3253 is a dual 1-of-4 channel high-speed FET analog multiplexer
in a 16-pin QFN package. Please refer to the TI SN74CBTLV3253 website at
© Kionix 2007
Date
Page 4 of 6
AN 003
http://focus.ti.com/docs/prod/folders/print/sn74cbtlv3253.html for product
summaries, power requirements, specifications, and schematics.
When multiplexing the KXPC4, only three of the eight input channels and one
corresponding output channel are needed. Data flow of a specified output on pin
Vmux can be attained by appropriately toggling the select pins (S1, S0) with the
system microprocessor. The schematic in Fig. 3 shows the recommended
connections to the SN74CBTLV3253.
Fig. 3) KXPC4/SN74CBTLV3253 Schematic
As previously stated, the system microprocessor will need to toggle the multiplexer
select pins (S0, S1) to obtain the desired accelerometer output. The
SN74CBTLV3253 output select table below shows the select pin inputs required to
obtain a specified output.
SN74CBTLV3253 Output Select
S0
S1
Vmux
0
0
1
0
1
0
Output X
Output Y
Output Z
SN74CBTLV3253 Output Select Table
© Kionix 2007
Date
Page 5 of 6
AN 003
System Sampling Rate
If the multiplexed KXPC4 is integrated with a microprocessor such as the TI
MSP430F149, a maximum estimated data sampling rate can be computed. As
previously stated, this sampling rate will be gated by the MSP430’s A-D conversion
time. With the MSP430’s internal clock operating at 5MHz, this system can retrieve
and convert a reading from all three accelerometer outputs in about 15.6µs. At this
speed, the data sampling rate for this multiplexed KXPC4 system is approximately
64K tri-axis readings per second. This is more than sufficient for the KXPC4, which
will be operating with a typical bandwidth of ~50Hz and a maximum of 3000Hz.
Theory of Operation
Kionix MEMS linear tri-axis accelerometers function on the principle of differential
capacitance. Acceleration causes displacement of a silicon structure resulting in a
change in capacitance. A signal-conditioning CMOS technology ASIC detects and
transforms changes in capacitance into an analog output voltage which is
proportional to acceleration. These outputs can then be sent to a micro-controller
for integration into various applications. Kionix technology provides for X, Y and Zaxis sensing on a single, silicon chip. One accelerometer can be used to enable a
variety of simultaneous features including, but not limited to:
Drop force modeling for warranty management
Hard disk drive shock protection
Tilt screen navigation
Theft, man-down, accident alarm
Image stability, screen orientation
Computer pointer
Navigation, mapping
Game playing
For product summaries, specifications, and schematics, please refer to the Kionix
accelerometer product sheets at http://www.kionix.com/sensors/accelerometerproducts.html.
© Kionix 2007
Date
Page 6 of 6
Similar pages