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Chapter 12
Instrumentation: Sensors to A/D Converters
Ron Mancini
12.1 Introduction
The typical transducer measurement system block diagram is shown in Figure 12–1. The
transducer is the electronic system’s interface with the real world, and it issues data about
a variable. The transducer converts the data into an electrical signal adequate for processing
by the circuitry that follows the transducer. Bias and excitation circuitry does the
care and feeding of the transducer, thus this circuitry provides offset voltages, bias currents,
excitation signals, external components, and protection that is required for the
transducer to operate properly. The output of the transducer is an electrical signal representing
the measured variable.
Input
Variable
Transducer Amplifier ADC
DIgital
Data
Out
Bias or
Excitation
Circuits
Power
Supply
Digital
Control
Figure 12–1. Block Diagram of a Transducer Measurement System
The variables that must be measured are determined by the customer’s application, and
the measured variable normally dictates the transducer selection. If the measured variable
is temperature, then some sort of temperature sensing transducer must be
employed, and the range of temperatures to be measured or the accuracy of the measurement
is the primary factor influencing temperature transducer selection. Notice that the
electrical output of the transducer is not a major concern at this point in the transducer
selection. The transducer’s electrical output is always a consideration, although picking
the right transducer for the job is the primary goal. The correct transducer for the job can
have an Ω/°C change, µV/°C change, or mV/°C change. All transducers have offset voltages
or currents, and they can be referenced to ground, either power supply rail, or some
other voltage. The selection of the transducer is out of circuit designer’s hands; thus, the
circuit designer must accept what the application demands.
12-1