Analog electronic process transmitters typically have only two calibration adjustments: one for zero and another for span. Occasionally you may find an analog electronic transmitter with a third adjustment: one for linearity. Modern “smart” process transmitters have more components in need of adjustment. A block diagram of a typical smart pressure transmitter shows this very clearly: Smart Transmitter The purpose of the analog-to-digital converter (ADC) is to translate the pressure sensor’s electrical output signal into a digital number the microprocessor can understand. Likewise, the purpose of the digital-to-analog converter (DAC) is to translate the digital output of the microprocessor into a 4 to 20 mA DC current signal representing measured pressure. The procedure of calibrating the ADC is called a sensor trim, while the process of calibrating the DAC is called an output trim. Explain the importance of performing both a sensor trim and an output trim whenever calibrating a “sm...
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