SLOA034 - Datasheet Archive

 

SLOA034 - September 1999 Signal Conditioning Wheatstone Resistive Bridge Sensors James Karki Mixed Signal Products ABSTRACT

 

Application Report SLOA034 SLOA034 - September 1999 Signal Conditioning Wheatstone Resistive Bridge Sensors James Karki Mixed Signal Products ABSTRACT Resistive elements configured as Wheatstone bridge circuits are used to construct force and pressure sensors. The resistive elements used to make the bridge change resistance in response to mechanical strain. This report discusses the basic concepts of resistive bridge sensors and three circuits commonly used for signal conditioning their output. Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Signal Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Single Op Amp Differential Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Three Op Amp Instrumentation Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Two Op Amp Instrumentation Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 3 4 List of Figures 1 2 3 4 5 6 1 Wheatstone Bridge Sensor and Thevenin Equivalent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single Op Amp Differential Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensor Output With No Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thevenin Equivalent Shows Required Gain is 50% Higher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three Op Amp Instrumentation Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two Op Amp Instrumentation Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3 3 3 4 4 Introduction There are two main technologies used to create resistive bridge sensors: strain gauge (or gage) and integrated circuits. Strain gages are widely used and have been available for many years. Typically, the strain gauge is bonded to a rigid structure, and when a force acts upon the structure, the strain gauge changes resistance. Strain gauge sensors are commonly used for both force and pressure measurement. For more details refer to The Pressure Strain and Force Handbook, Omega Engineering, Inc. http://www.omega.com. More recently, monolithic resistive bridge sensors have become commercially available as integrated circuits. The bridge circuit is constructed on a silicon die. When a force is exerted on the die, the resistance changes. Normally this type of sensor is used to measure pressure. For more information refer to Solid­State Pressure Sensors Handbook, Sensym, http://www.sensym.com. 1 Figure 1 shows a resistive Wheatstone bridge circuit and its Thevenin equivalent. When an excitation voltage is applied between Vexc and GND and all resistances are equal, the voltage at SIG+ and SIG­ is 1/2 Vexc. Thevenin Equivalent Wheatstone Bridge Sensor R+R Sig+ R­R Sig- VTH+ + ­ Sig+ R­R R