Adjustment-free Inclinometer Operates On +2.7v

Maxim Integrated Products, Inc.

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Adjustment-Free Inclinometer Operates On +2.7V
Jul 09, 1998
APPLICATION NOTE 1048
Adjustment-Free Inclinometer Operates On +2.7V
Figure 1
is an inclinometer (tilt-measuring circuit) whose Sensor (SN1) is filled with liquid electrolyte. Acting as
a Potentiometer the inclinometer produces a voltage proportional to tilt on its center electrode. Because the
liquid is subject to electrolysis, the sensor's forcing voltage must be AC with an average DC component of zero.
IC1 is an 8-channel, 12-bit analog-to-digital converter (ADC) that digitizes the Sensor output for use by IC2, the
Figure 1. This tilt Sensor is simple, accurate, inexpensive, and adjustment free.
Conditioning circuitry for this Sensor type usually includes op amps, analog Switches and Potentiometers
Because Potentiometer settings drift with time and temperature, such systems require periodic recalibrations
based on a precise and tedious procedure. The synchronous approach shown in Figure 1 not only eliminates the
need for calibration, but it also operates from a single-supply voltage as low as +2.7V.
Two CMOS port pins on the µC generate 50Hz square waves, 180° out-of-phase, as an AC drive for the Sensor
When the Sensor is level, its center-electrode voltage (filtered by R3/C4 and fed to the ADC) is midway between
these drive-electrode voltages, which are approximately V
CC
and 0V. Each port pin has a finite resistance and
resultant voltage drop. To compensate for the resulting inaccuracies, voltage divider R4/R5 samples the drive
signal's mid-level voltage and feeds it to channel 2 on the ADC This voltage remains constant, but the center-
electrode signal varies above or below mid-level according to the direction of tilt.
The tilt signal on one channel and the reference (mid-level) signal on another are digitized by the ADC and fed to
the µC. The AC drive dwells 10ms on each polarity, allowing about nine time constants for 12-bit settling before
the A/D conversion. The converter's pseudo-differential input negates the absolute value of these signals