Pressure sensors can come in a variety of technologies, such as piezoresistive, capacitive, electromagnetic etc. Piezoresistive pressure sensors are the most commonly used of this group.
In this four part series, piezo-resistive pressure sensing basics and the PSoC circuits for three types of pressure sensors will be examined. The first part covers piezo-resistive pressure sensor basics and introduces three categories of pressure sensors
Piezo-resistive Pressure sensor basics
A piezo resistive pressure sensor has a silicon diaphragm whose resistance depends on its tension. The diaphragm undergoes tension whenever there is a pressure. It can be modelled by a Wheatstone s bridge where all the resistors change with pressure. When pressure is applied to the diaphragm, resistance of the two arms (diametrically opposite to each other) increases and the resistance of the other two arms decreases.
Pressure sensor equations
The change in resistance can be converted to voltage by voltage or current excitation. The equations involved in voltage and current excitation are shown below.
Voltage Excitation Mode:
In this case, the Wheatstone s bridge is excited by a voltage. Span is defined as the bridge output voltage for rated pressure (full pressure). Span( S) is given by
S = V * R/R
R Change in resistance for rated pressure
R - Bridge resistance
V Excitation voltage
R = P * Ps
P Rated Pressure
Ps Pressure sensitivity (Change in resistance for unit change in pressure)
Ps = R * k
k - Normalized pressure sensitivity i.e. Pressure sensitivity for 1ohm resistor
S = V * P * k
Span is independent of bridge resistance. The temperature coefficient of span primarily results from the temperature coefficient of pressure sensitivity which is dependent on the diaphragm material.
Current excitation:
In this case, the bridge is excited by a current source. In this case span is given by,
S = I * R * P * k
Where I is the excitation current.
In this case, the span depends on the current source and bridge resistance.
The temperature coefficient of span results from the temperature coefficient of resistance and the temperature coefficient of pressure sensitivity. By proper design, the two can be made close to each other. Hence current excited pressure sensors have the design advantage of tweaking the process parameters so as to reduce the effect of temperature on span.
Pressure sensor types
The pressure sensor span is generally around 50-150mV. Depending on whether the pressure sensor output is amplified and on whether the pressure sensor is compensated for temperature variations of span and offset, we can have the following categories of pressure sensors
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Unamplified uncompensated pressure sensors
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Unamplified compensated pressure sensors
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Amplified pressure sensors/transmitters.
The next three parts explains interfacing each type of pressure sensor with PSoC and the system performance measures.
By Praveen Sekar