Fundamentals of Industrial Instrumentation and Process Control, Second Edition by William C. Dunn

Author:William C. Dunn [Dunn, William C.]
Language: eng
Format: azw3
Publisher: McGraw-Hill Education
Published: 2018-09-27T16:00:00+00:00

9.5.1 Offset Zero

Figure 9.15a shows the output of a sensor when measuring a variable and the idealized output obtained from a linearization circuit with adjustment of the bias (zero level) as is required on many types of sensor outputs. The output voltage from the linear operating range of the sensor varies from 0.35 to 0.7 V as the process variable varies from low to high over its measurement range. However, the sensor output goes to equipment that requires a voltage from 0 to 3.5 V for the range of the variable. A circuit for changing the output levels is shown in Fig. 9.15b. The reference input to the amplifier is set at 0.35 V to offset the minimum level of the sensor to give zero out at the low end of the range. The gain of the amplifier is set to 10 (with feedback resistors 47k/4.7k) giving 3.5 V output with 0.75 V input, i.e., 3.5/(0.7 − 0.35) = 10. Note the use of impedance matching buffers that would be used in instrumentation.

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