ADXL278: Self test pin

Document created by analog-archivist Employee on Feb 23, 2016
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The datasheet states that the selftest input may be used to verify the function
of the full signal path. But it does not state whether to apply a high or low
to the pin to activate it. what is the voltage on the pin for normal operation?


The self test feature designed in all of our inertial sensors activates the
mechanical structure of the device which in turns generates a signal through
the electronic portion of the chip, thereby allowing the user to verify proper
operation of the entire device.  By applying a logic 1 signal on the ST pin, a
voltage is applied on special self test 'fingers' located around the moving
element of the sensor.  This voltage creates an electrostatic force which
causes the structure to move.  The sensor displacement generates an electrical
signal on the sensing fingers, as if the sensor were moved under normal
acceleration.  The signal is processed through the signal conditioning
circuitry and appears at the output of the device.  The self test feature
therefore exercises every part of the chip.  A proper output signal is
therefore indicative that the entire sensor is functional. 

The self test feature cannot be used to verify drift of the device.  It should
not be used to verify parametric compliance.  It should only be used to verify
functionality.  If changes such as drift over temperature cannot be tolerated
in the application, these need to be addressed through calibration or
temperature compensation routines.

The internal pull down is 30k minimum to 50k maximum. The turn on level is
0.8*Vdd maximum and the turn off level is 0.2*Vdd minimum. We do not test or
specify the minimum turn on level or maximum turn off level.

Maximum voltage at ST pin = Vdd + 0.2V