ELECTENG 101 Chap.3 Sensors and Interface Circuits
Sensors and Interface Circuits
Resistive transducers turn temperature, strain or another physical quantity into a resistance change. Divider and Wheatstone-bridge interfaces turn that change into a voltage that later stages can process. The important habits are predicting direction before calculation, keeping the sensor law separate from the interface law and checking whether the receiving circuit loads the output.
A sensor characteristic first maps the measurand to resistance; the electrical network then maps resistance to voltage. Changing whether the sensor occupies the upper or lower divider leg reverses the output trend. A bridge compares two midpoint voltages and can reject a large common baseline while exposing a small imbalance. Its output sign depends on the declared subtraction order.
Any receiver with finite input resistance alters the interface, so the unloaded prediction is only a reference point rather than the final measured result.
What this chapter covers
- 01
Resistive transducer laws
- 02
Divider transfer direction
- 03
Sensitivity around an operating point
- 04
Wheatstone bridge balance
- 05
Differential output polarity
- 06
Receiver loading
Worked example · free
A bridge moves away from balance
- +1Both midpoint voltages begin at 2.5 V.
- +3The changed midpoint is 5×2.2/4.2 = 2.619 V.
- +2Output is +0.119 V for right minus left.
Key terms
- Resistive transducer
- A sensor whose resistance changes with the physical quantity being measured.
- Sensitivity
- The change in electrical output per unit change in the measured input near an operating point.
- Bridge balance
- The condition where two divider midpoint voltages are equal and differential output is zero.
Sensors and Interface Circuits FAQ
Why predict the divider direction first?
The prediction detects a swapped numerator or reversed sensor position. A lower-leg resistance increase raises the output fraction, while an upper-leg increase lowers it. Writing that direction before substituting numbers provides an independent check: if the arithmetic moves the other way, inspect the circuit placement and chosen output terminals instead of trusting the calculator.
What does a bridge measure?
A Wheatstone bridge compares two divider midpoint voltages. It is especially useful for detecting a small resistance change around a balanced operating point. At balance the arm ratios are equal and the differential output is zero, even though current still flows through both divider branches. Leaving balance gives both magnitude and sign information about the change.
How does loading affect a sensor interface?
Finite receiver resistance changes the effective divider leg and therefore the output. Compare input resistance with source resistance or buffer the signal when the error is too large. Calculate the parallel combination created by the receiver before applying the divider law, and report loading error relative to the open-circuit interface voltage so the design consequence is explicit.
Why separate the sensor law from the interface law?
The sensor law relates temperature, strain or another physical input to resistance, whereas the interface law relates that resistance to an electrical output. Keeping two stages separate makes units and assumptions auditable. It also lets a different divider, bridge or amplifier be attached without rewriting the physical transducer model.
What determines bridge output polarity?
Polarity comes from the stated order of midpoint subtraction. Calculate each midpoint relative to the same reference and subtract in that order. Swapping the measurement leads reverses the sign but not the imbalance magnitude, so a complete answer names which midpoint is higher rather than presenting an unsigned voltage alone.
When should a sensor interface be buffered?
Buffer the interface when the following stage has insufficient input resistance, when several loads share the signal, or when a small bridge output must be preserved before amplification. A voltage follower ideally draws negligible input current and offers a low output resistance, but its supply rails and input range still constrain real operation.
Exam move
Draw the sensor, fixed resistor, supply and output polarity for every problem. First calculate the resistance produced by the physical input. Then apply the interface equation and only afterward include loading. For bridges, calculate each midpoint separately and subtract in the stated order.
Drill qualitative questions that ask only whether output rises or falls because they expose topology understanding without hiding behind arithmetic. Make a two-stage table for every example: physical quantity to resistance, then resistance to output voltage. Vary the sensor between upper and lower divider positions and predict the trend without calculating.
Next use the same resistance change in one bridge arm, reverse the output lead order, and explain why only the sign changes. Add a receiver resistance and compare unloaded voltage, loaded voltage and percentage error. When the error is unacceptable, sketch where a buffer belongs and identify its input and output resistance goals.
Finish with a tolerance case so nominal balance, sensitivity and practical offset are treated as distinct design questions.
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