CIRCUIT060094

Some MSP430™ microcontrollers (MCUs) contain configurable integrated signal chain elements such as op-amps, DACs and programmable gain stages. These elements make up a peripheral called the smart analog combo (SAC). For information on the different types of SACs and how to leverage their configurable analog signal chain capabilities, visit MSP430 MCUs smart analog combo training. To get started with your design, download the strain gauge bridge amplifier circuit design files. A strain gauge is a sensor whose resistance varies with applied force. The change in resistance is directly proportional to how much strain the sensor is experiencing due to the force applied. This pressure sensing circuit uses a strain gauge placed in a bridge configuration to measure the variation in resistance. This design leverages all four of the built-in op-amp blocks (SACs) in the MSP430FR2355. Two SAC_L3 peripherals are configured in general-purpose mode to amplify a differential signal created by the change in resistance of a strain gauge while the other two are configured in DAC mode to supply the reference voltage (Vref) and the excitation voltage (Vex). By varying R10, a small differential voltage is created at the output of the Wheatstone bridge which is fed to the two SAC op-amp instrumentation amplifier inputs. The linearity of the instrumentation amplifier is based on the input common-mode and output-swing ranges of the MSP430 SAC op-amp, which can be found in the specification chart at the end of this document. The output of the second stage op-amp can be sampled directly by the on-board ADC or monitored by the onboard comparator for further processing inside the MCU.