Sensors & Measurement · Study deck

Sensor Circuits: Conditioning and Filters

This first route shapes small sensor signals with amplification, filtering, protection, and calculated interfaces.

Physics Phoebe is your guide for this deck.

sensorcircuitssignal

Physics Phoebe, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Design Signal Conditioning Chains: Construct complete pipelines from raw sensor output to ADC-ready signals
Calculate Amplifier Gain: Derive required gain to utilize full ADC range for maximum resolution
Implement Wheatstone Bridges: Configure bridge circuits for precision resistance measurements
Evaluate Worked Examples: Trace step-by-step calculations for real thermistor and strain gauge interfaces

Major section

Start With the Measurement Story

Protect the Meaning of a Weak Signal: An analog-to-digital converter is a part that turns a voltage into a number.

Its useful change is tiny, while motors, long wires, heat, and the power supply can add much larger unwanted changes.
Switch nearby loads, change cable position, and apply a value just beyond the safe limit.
A clean number is not proof if the chain hid a fault.

Major section

Start With the Measurement Story (continued)

One bench result cannot cover every part, temperature, or installation.

The deeper sections explain bridges, amplifiers, filters, and conversion maths so each stage keeps a stated limit and test.
A raw sensor output is rarely ready for a dashboard or control loop.
The first design move is to identify what must be amplified, filtered, level-shifted, isolated, or protected before the reading can be trusted.

Major section

Signal Conditioning Chain

This four-stage pipeline is the foundation of all professional sensor interfaces.

Its 16 kohm, 1 uF low-pass has $f_c=9.95$ Hz and attenuates 60 Hz by 15.7 dB; gain, filtering, and buffering therefore protect different parts of the same measurement chain.

Major section

Signal Processing Pipeline Overview

The physical quantity is transduced into an electrical signal; conditioning scales and filters it before conversion; the ADC produces codes; digital processing turns codes into evidence; and the output stage uses or transports the result.

The stage map explains where work happens, but a practical design must also decide what to trade.

Complete Sensor Signal Processing Pipeline: Physical to Digital to Action

Major section

Wheatstone Bridge

The Wheatstone bridge is a precision measurement circuit used with strain gauges, load cells, and other resistive sensors requiring high accuracy.

It detects small resistance changes by comparing two voltage dividers.
The bridge makes the problem visible: a real physical change may only move the output by millivolts.
The visual exposes the two divider branches whose midpoint voltages are compared.

Wheatstone Bridge: Four-resistor configuration for detecting small resistance changes with high precision.

Major section

Signal Conditioning Chain

Enter a practical resistor and inspect the calculated ideal capacitance alongside its nearest E12 value; the standard component changes the realised cutoff, so use that realised value for the sampling check.

This connects the component choice to the acquisition boundary rather than treating the calculator as a parts lookup.

Signal conditioning prepares a small sensor signal for ADC conversion through gain, filtering, level shifting, and sampling.

Major section

HVAC Thermistor Conditioning

Scenario: You need to measure room temperature (15-35C) using an NTC thermistor for a smart HVAC system.

The thermistor output is only 20mV at 25C, but your ESP32 ADC needs 0-3.3V input.
Package pins and circuit limits remain part of the design even when the arithmetic is correct.

A gold metal-can AD520J instrumentation amplifier integrated circuit with two rows of pins

Deck summary

Key takeaways

Protect the Meaning of a Weak Signal: An analog-to-digital converter is a part that turns a voltage into a number.

One bench result cannot cover every part, temperature, or installation.
This four-stage pipeline is the foundation of all professional sensor interfaces.
The physical quantity is transduced into an electrical signal; conditioning scales and filters it before conversion; the ADC produces codes; digital processing turns codes into evidence; and the output stage uses or transports the result.
The Wheatstone bridge is a precision measurement circuit used with strain gauges, load cells, and other resistive sensors requiring high accuracy.

Retrieval practice

Recall check

Physics Phoebe says: answer from memory, then check your reasoning.

Q1A strain gauge Wheatstone bridge is balanced at zero load with all four 350ohm resistors. Under maximum load, one gauge changes by 0.1% (to 350.35ohm). With 5V excitation, what differential output voltage do you expect?

A0V - the bridge is still balanced

BApproximately 1.25mV

CApproximately 12.5mV

DApproximately 5mV

Show answer

Answer: B Correct!

Q2A pressure sensor outputs 0-50mV for 0-100 PSI range. You want to interface it with a 3.3V, 12-bit ADC. What amplifier gain provides the best resolution while staying within the ADC input range?

AGain of 10 (output 0-500mV)

BGain of 66 (output 0-3.3V)

CGain of 100 (output 0-5V)

DGain of 1 (direct connection)

Show answer

Answer: B Correct!

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Answers

Answer key.

B · Correct!
B · Correct!