ASACK0021

AN 021
Self Test Response
Introduction
Self Test is a standard feature in Kionix MEMS accelerometers that enables our
customers to verify that the part is functional. However the customer must use the
proper algorithm to ensure functionality. Applicable theories, plots, and equations
are provided with this note as guidelines.
Self Test Function
The accelerometer’s self test is activated when ‘logic high or 1’ is applied to the ST
pin or when commanded digitally via I2C or SPI. Once activated, an electrostatic
force is applied to the sense element that causes the mass to move and the outputs
to change, as shown in Figure 1. The product specification states the expected
amount of output change. The self test function exercises the sense element and
portions of the ASIC. The output change of the self test function is modified by the
internal Low Pass Filter (LPF), where applicable, and any external low pass filter.
Selftest activation
Selftest output
Figure 1: Selftest Function for KXR94 ±1.5g
Figure 1. An example of the self test output with a 50Hz external LPF. The self test
is actuated at 10 Hz and 25% duty cycle.
36 Thornwood Dr. – Ithaca, NY 14850
tel: 607-257-1080 – fax:607-257-1146
www.kionix.com - [email protected]
© Kionix 2007
3 May 2007
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Recommended Self Test Algorithm
1. Check the outputs under normal operation. At this time, the carrier voltages
on the sense element are balanced so the only force on the sensor is from
acceleration.
2. Self test sequence
a. A microcontroller activates self test by applying a voltage greater than
0.8*Vdd to the ST pin or by turning on the ST bit in the appropriate
control register.
b. The carrier voltages are adjusted to create a net electrostatic force on
the sense element.
c. The sense element is
capacitance (∆C) occurs.
deflected
and
a
subsequent
change
in
d. This ∆C is amplified and converted to an output voltage by the charge
amplifiers of the ASIC. For a digital part, this voltage is fed into an
analog-digital converter.
e. Measure the outputs after a delay determined by the bandwidth of the
LPF. Note this response is superposed on any acceleration.
f.
Deactivate self test by applying a voltage less than 0.2*Vdd to the ST
pin or by turning off the ST bit in the appropriate control register. The
accelerometer outputs return to normal operation after the appropriate
delay time.
3. Compute the self test response. The output measured with self test off is
subtracted from the output measured with the self test on. This delta is
divided by the sensitivity of the part to compute the self test response in g’s,
as shown in the following equation:
ST (in g ) =
(OutputSTon − OutputSToff )
Sensitivity
This self test response should be compared to the product specification to
determine if the product is functioning correctly.
© Kionix 2007
Date
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Output response for the KXR94
5
4.5
ST Response from ASIC is
shown – Normal +1g
4
Output V / g level change
3.5
ST Response from ASIC is
shown – Normal -1g
3
2.5
2
1.5
1
0.5
0
-0.01
0
0.01
0.02
time seconds)
0.03
0.04
g level response
data2
data3
Response from +1g
data5
data6
0.05
Response from -1g
data8
data9
Figure 2. An example of the output response for the KXR94. In this example, the
KXR94 is operating at 5V with a sensitivity of 1.33 V/g
Response Time
The self test response time is determined by the low pass filter on the outputs. For
an RC filter, it takes 1 time constant (τ) to achieve 63.2% of the voltage change. As
shown in the figure below, it takes 5 time constants to achieve 99.3% of the output
voltage change.
Figure 3. The output response of an RC filter to a step function change in input
voltage
© Kionix 2007
Date
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AN 021
Thus, the response time is defined as 5 RC time constants. The table below shows
typical LPF cutoff frequencies and their associated response times.
LPF cutoff frequency
RC time constant
Response time
1000 Hz
0.16 ms
0.8 ms
500 Hz
0.32 ms
1.6 ms
250 Hz
0.64 ms
3.2 ms
100 Hz
1.6 ms
8 ms
50 Hz
3.2 ms
16 ms
25 Hz
6.4 ms
32 ms
10 Hz
16 ms
80 ms
When performing self test checks on the accelerometer, it is critical that the
measurements after activation or de-activation are delayed by a time greater than or
equal to the response time of the output filter.
Self Test Response Specifications
The self test response of each accelerometer that Kionix has to offer varies from
product to product. For a given product, the self test response will vary greatly with
operating voltage. For this reason in the product specifications you will be able to
find the typical self test response along with the min/max self test response values
so you can verify that the product is working properly. This feature is a plus for
customers that are not able to flip or rotate to verify that the accelerometer is
functioning within specification.
© Kionix 2007
Date
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The Kionix Advantage
Kionix technology provides for X, Y, and Z-axis sensing on a single, silicon chip. One
accelerometer can be used to enable a variety of simultaneous features including,
but not limited to:
Hard Disk Drive protection
Vibration analysis
Tilt screen navigation
Sports modeling
Theft, man-down, accident alarm
Image stability, screen orientation & scrolling
Computer pointer
Navigation, mapping
Game playing
Automatic sleep mode
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. For product summaries, specifications, and
schematics, please refer to the Kionix MEMS accelerometer product sheets at
http://www.kionix.com/sensors/accelerometer-products.html.
© Kionix 2007
Date
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