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Keywords:
Linearization of Wheatstone Bridge, bridge circuits, RTD, temperature detection, temperature monitoring,
analog front end for temperature sensing, Precision amplifier, Precision voltage reference, MAX44267, MAX6025,
industrial, medical
APPLICATION NOTE 6144
LINEARIZATION OF WHEATSTONE-BRIDGE
By:
Ashwin Badri Narayanan, Member of Technical Staff, Maxim Integrated
Abstract: This application note discusses the resistance-variable element in a Wheatstone bridge—the first choices
for front-end sensors. We will examine its behavior and explain how to linearize the bridge circuit to optimize
performance. The simplicity and effectiveness of a bridge circuit makes it very useful for monitoring temperature,
mass, pressure, humidity, light, and other analog properties in industrial and medical applications.
Introduction
Wheatstone bridge circuits have been in the field for a very long time and still are among the first choices for front-
end sensors. Whether the bridges are symmetric or asymmetric, balanced or unbalanced, you can accurately
measure an unknown impedance using the bridge. In fact, the simplicity and effectiveness of a bridge circuit makes it
very useful for monitoring temperature, mass, pressure, humidity, light, and other analog properties in industrial and
medical applications.
The Wheatstone bridge has a single impedance-variable element that is inherently nonlinear away from the balance
point. Bridge circuits are commonly used to detect the temperature of a boiler, chamber, or a process situated
hundreds of feet away from the actual circuit. Usually a sensor element, typically a resistance temperature detector
(RTD), thermistor, or thermocouple, is situated in the hot/cold environment to provide information about resistance
change to temperature.
In the following discussion, we will consider this resistance-variable element in a Wheatstone bridge. We will
examine its behavior and explain how to linearize the bridge circuit to optimize performance. Note finally, that when
we speak generally about "bridges," this article is focused on circuit design for a Wheatstone bridge.
Single Variable-Resistance Wheatstone Bridge
Resistance-variable Wheatstone bridge circuits perform most of the front-end tasks in a design. They use
inexpensive, accurate discrete parts. By incorporating an RTD element, the bridge's inherent resistance variations
are kept within the accepted linearity and tolerance limits, depending on the manufacturer of the RTD.
RTD devices have a very detailed data sheet characterizing their behavior with look-up tables and even transfer
function equations down to four or more orders of error compensating terms. To ensure a high-precision system,
designers must consider both the inherent nonlinearity of the RTD element and the Wheatstone bridge, then painfully
calibrate the front-end, and linearize the front-end at the microcontroller side. Increasing the order of the equation in
the microcontroller is going to improve the linearity. A typical bridge circuit (
Figure 1) detects milliohms of changes
in resistance (ΔR).
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