Page 368 - The Mechatronics Handbook

FIGURE 18.5
Examples of accuracy and precision.

average accuracy is poor. The second set of data shows a low degree of precision (high standard deviation),
but the average accuracy is good. The third set of data shows both low precision and low accuracy, while
the fourth set of data shows both high precision, high repeatability, and high accuracy.

18.7 Impedance

Impedance is the ratio of voltage and current flow for a sensor. For a simple resistive sensor (such as a
strain gage or a thermistor), the impedance Z is the same as the resistance R, which has units of ohms (W),

V
Z R = --- = R
I
For more complicated sensors, impedance includes the effects of capacitance, C, and inductance, L.
Inclusion of these terms makes the impedance frequency sensitive, but the units remain ohms:

V 1 V
Z C = --- = ----------- and Z L = --- = jLω
jCω
I
I
where j = – 1 is the imaginary number and ω is the driving frequency. The impedance form is
particularly nice for analyzing simple circuits, as parallel and series inductances can be treated just like
resistances. Two types of impedance are important in sensor applications: input impedance and output
impedance. Input impedance is a measure of how much current must be drawn to power a sensor (or
signal conditioning circuit). Input impedance is frequently modeled as a resistor in parallel with the

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