Dataforth Application Note - Basic Bridge Circuits

March 2018, MARIETTA, GA ~

Preamble
Bridge circuits have been in use for well over 150 years. To date, the bridge is still the most economical circuit technique for accurately measuring resistance. The original bridge circuit topology has had many unique modifications and has been applied to applications such as AC measurements, automatic balancing, oscillators, and amplifiers. Perhaps the best known application for Samuel Hunter Christie’s circuit is the bridge strain gage for strain type measurements in mechanical assemblies and building structures.

This application note will focus primarily on some subtleties of bridge circuit excitation and associated performance. Analysis of all the many bridge topologies that compensate for small second order effects are beyond the scope of this application note. Readers interested in the in-depth details of complex bridge topologies and strain gage applications should explore the internet, which contains thousands of sites dedicated to such details. In addition, readers are encouraged to examine Dataforth’s complete line of Signal Conditioning Modules (SCMs) dedicated to strain gage bridge applications, Reference 1.

Basic Bridge Circuits
The following examples focus only on Figure 1 type bridge circuit topologies with a single resistive variable element. Output responses, including the effects of excitation line resistance for both voltage and current bridge excitation as well as bridge linearity, are examined. Errors due to resistances of poorly made contacts and corrosive action of dissimilar metals are neglected. Moreover, the output line resistances are neglected since it is standard practice to measure bridge output voltages with high impedance (typically > 1MÙ) devices.

Analytical investigations throughout this document focus on the R-ohm type bridge, which means all bridge resistors are “R” ohms when not exposed to the field process variables. Figure 1 represents the R-ohm bridge type field sensor with all bridge resistors (R1, R2, R3, Rx) located at the point of field measurement; however, as resistor Rx is the bridge resistive sensor element, it varies with process parameters such as temperature, flow, pressure, level, humidity, strain, etc. In R-ohm bridge topologies, R1, R2, R3 are equal to R and Rx = (R+Ä R) where Ä R is a function of process variables.

Examples
Two categories of R-ohm bridge topologies will be examined. Category 1 bridges are defined as bridge topologies with all bridge resistors located in the field with one or more elements exposed to the process variable; Category 2 bridges are ...

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