Sensors and Signal Conditioning

Modified: Mar. 14, 2026

Published: Mar. 10, 2026

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A sensor converts a physical quantity (temperature, light, pressure, strain) into an electrical signal. But that raw signal is rarely ready for a microcontroller’s ADC. It might be too small (millivolts), too noisy, have a DC offset, or change with the wrong polarity. Signal conditioning is the art of transforming the raw sensor output into a clean signal that fills the ADC’s input range and accurately represents the measured quantity. This lesson ties together everything you have learned in this course. #AnalogElectronics #Sensors #SignalConditioning

The Signal Conditioning Chain

Every sensor-to-digital conversion follows a similar path:

1. Sensor Converts a physical quantity into an electrical signal (resistance change, voltage, current).

2. Excitation Provides a stable voltage or current to the sensor (voltage divider, constant current source, Wheatstone bridge).

3. Amplification Increases the signal amplitude to match the ADC input range (op-amp, instrumentation amplifier).

4. Filtering Removes noise and prevents aliasing (RC low-pass filter, active filter).

5. Protection Limits the voltage to the ADC’s safe input range (clamping diodes, Zener, series resistor).

6. ADC Sampling Converts the conditioned analog signal to a digital value for processing by the MCU.

Each stage uses concepts from the previous lessons in this course. This lesson shows how they combine into a complete measurement system.

Common Sensor Types

Resistive Sensors

These sensors change their resistance in response to a physical quantity:

Resistive sensors are read by placing them in a voltage divider or Wheatstone bridge and measuring the resulting voltage.

Voltage-Output Sensors

Some sensors produce a voltage directly:

These typically require amplification and possibly level shifting before ADC input.

Thermistor Circuits

The NTC (Negative Temperature Coefficient) thermistor is the most common temperature sensor for hobbyist and embedded projects. Its resistance decreases as temperature increases, following an exponential relationship.

Thermistor in a Voltage Divider

The simplest thermistor circuit uses a fixed resistor and the thermistor in a voltage divider:

    VCC (3.3V)
      |
    [Rf] 10K (fixed)
      |
      +------> Vout (to ADC)
      |
    [NTC] 10K at 25C
      |
     GND

  Cold (0C):  NTC=27K, Vout=2.42V
  Room (25C): NTC=10K, Vout=1.65V
  Hot (50C):  NTC=3.6K, Vout=0.87V

When the thermistor is cold (high resistance), is close to . When hot (low resistance), drops toward 0V.

Choosing the Fixed Resistor

For maximum sensitivity, choose the fixed resistor equal to the thermistor’s resistance at the midpoint of your measurement range. For a 10k NTC thermistor at 25 degrees Celsius:

• At 25C: ,
• At 0C: ,
• At 50C: ,

A 10k fixed resistor gives a useful voltage swing of about 0.47 over the 0 to 50C range.

Steinhart-Hart Equation

The relationship between thermistor resistance and temperature is non-linear. The Steinhart-Hart equation provides an accurate model:

Where:

• is temperature in Kelvin
• is thermistor resistance in ohms
• , , are coefficients specific to the thermistor (from the datasheet)

A simpler (less accurate) approximation uses the B-parameter:

Where (25C), (resistance at ), and for a typical 10k NTC.

This calculation is typically done in software on the MCU after the ADC reads the voltage.

Light Dependent Resistor (LDR)

An LDR (photoresistor) changes resistance with light intensity. In darkness, resistance is very high (1M ohm or more). In bright light, resistance drops to a few hundred ohms.

LDR Voltage Divider

VCC ── R_fixed (10k) ──┬── V_out (to ADC)
                       │
                       LDR
                       │
                      GND

In bright light: LDR resistance is low, is low. In darkness: LDR resistance is high, is high.

Swapping the LDR and fixed resistor inverts this relationship.

LDRs are slow (response time 10 to 100 ms) and not very accurate, but they are cheap and easy to use for light/dark detection, automatic brightness control, and simple light meters.

The Wheatstone Bridge

For sensors with very small resistance changes (like strain gauges, which change by only 0.1 to 0.3%), a simple voltage divider lacks sensitivity. The Wheatstone bridge solves this by measuring the difference between two voltage dividers.

Bridge Circuit

            VCC (5V)
             |
       ┌-----+-----┐
       |            |
     [R1] 350     [R2] 350
       |            |
  Va --+            +-- Vb
       |            |
     [R3] Sensor  [R4] 350
     (strain gauge)  |
       |            |
       └-----+-----┘
             |
            GND

  Vbridge = Va - Vb
  Balanced: Va = Vb, Vbridge = 0
  Sensor change: small differential
  output (millivolts)

The bridge output voltage is the difference between the two midpoints:

When the bridge is balanced (), the output is zero. A small change in one resistor (the sensor) produces a small but measurable differential voltage.

Strain Gauge Example

A strain gauge has a nominal resistance of 350 ohm and a gauge factor of 2. Under a strain of 1000 microstrain ():

That is a change of 0.2%. In a voltage divider with a 5V supply, this produces a voltage change of only about 5 mV, which is easily overwhelmed by noise. In a Wheatstone bridge with a differential amplifier, this signal can be cleanly extracted and amplified.

Bridge Sensitivity

For a single active element (quarter bridge, where one resistor is the sensor and the other three are fixed):

For the strain gauge example:

This small signal requires careful amplification.

Instrumentation Amplifier

The instrumentation amplifier (INA) is a specialized high-gain differential amplifier designed for measuring small signals in noisy environments. It amplifies the difference between its two inputs while rejecting common-mode noise.

Three Op-Amp Instrumentation Amplifier

The classic instrumentation amplifier uses three op-amps:

• Two input buffer amplifiers (non-inverting configuration) provide high input impedance and initial gain
• One difference amplifier subtracts the two buffered signals and provides the final output

The gain is typically set by a single resistor ():

Where is a fixed internal resistor value.

Dedicated INA ICs

For production designs, dedicated instrumentation amplifier ICs are preferred over discrete op-amp implementations:

For breadboard prototyping with the LM358, you can build a simple differential amplifier (Lesson 5) that works adequately for signals above 10 mV. For microvolt-level signals (thermocouples, strain gauges), a dedicated INA is necessary.

Signal Conditioning Design Process

Here is a systematic approach to designing a signal conditioning circuit:

1. Characterize the sensor output What is the voltage range? What is the source impedance? Is the signal differential or single-ended? What is the frequency content?

2. Determine the ADC requirements What is the ADC input range (0 to 3.3V, 0 to 5V)? What is the ADC resolution (10-bit, 12-bit)? What is the maximum source impedance the ADC can handle?

3. Calculate the required gain

4. Design the amplifier stage Choose non-inverting for single-ended signals, differential/INA for bridge sensors. Select resistor values for the required gain.

5. Add filtering Place a low-pass filter after the amplifier with cutoff frequency at or below . This serves as the anti-aliasing filter.

6. Add protection Place series resistors (1k to 10k) and clamping diodes at the ADC input to protect against overvoltage. Many MCUs have internal clamping diodes, but external protection adds a safety margin.

7. Verify the design Measure the output at known sensor values. Compare with your calculated expected values. Check noise level with an oscilloscope if available.

Practical Build: Complete Temperature Measurement Circuit

This project combines a thermistor voltage divider, an op-amp buffer, a non-inverting amplifier, and an RC filter into a complete signal conditioning chain.

Components Needed

The Design

Goal: Read temperature from 0 to 100 degrees Celsius and output 0 to 3.3V for a 12-bit ADC.

Stage 1: Thermistor voltage divider

• 10k fixed resistor + 10k NTC thermistor
• At 25C:
• At 0C:
• At 100C:
• The voltage range is about 3.3V, which is convenient

Stage 2: Buffer (Op-amp A of LM358)

• Voltage follower isolates the thermistor from the amplifier

Stage 3: Gain stage (Op-amp B of LM358)

• Since the thermistor divider already produces approximately 0 to 3.3V over the 0 to 100C range, we can set the gain close to 1. For a narrower range (20 to 50C), we would increase the gain and add an offset.
• For this build, use gain = 1 (second buffer) to demonstrate the full chain cleanly

Stage 4: Anti-aliasing filter

• RC low-pass at the output: ,
• This is adequate for temperature measurement (which changes slowly)

  VCC         Buffer       Amplifier     Filter
   |        ┌------┐     ┌------┐
 [Rf]       |      |     |      |  [R]3.3K
   +------->|+ OUT-+---->|+ OUT-+--/\/\--+->ADC
   |        |      |     |      |        |
 [NTC]   +->|- ---┘   +->|- ---┘    [C]100nF
   |     |  └------┘   |  └------┘      |
  GND    |  (LM358 A)  |  (LM358 B)    GND
         |              |
      (output to     [R1]+[Rf pot]
       inv. input)    to GND

Build Steps

1. Build the regulated power supply Use the 3.3V regulator from Lesson 6, or a 5V USB supply. Place 100 nF decoupling capacitors at the supply points.

2. Build the thermistor divider Connect a 10k fixed resistor from VCC (use 3.3V if you want the output to stay within 3.3V range, or 5V and accept that some of the range will be above 3.3V) to a node. Connect the 10k NTC thermistor from that node to ground.

3. Connect the buffer Wire the divider output to pin 3 (non-inverting input A) of the LM358. Connect pin 1 (output A) to pin 2 (inverting input A). Power the LM358 with VCC on pin 8, GND on pin 4, and a 100 nF decoupling capacitor.

4. Connect the amplifier/second buffer Wire pin 1 (buffer output) to pin 5 (non-inverting input B). For a unity-gain buffer: connect pin 7 (output B) to pin 6 (inverting input B). For adjustable gain: connect a 10k resistor from pin 6 to GND, and a 10k potentiometer from pin 6 to pin 7.

5. Add the anti-aliasing filter Connect a 3.3k resistor from pin 7 (amplifier output) to the final output node. Connect a 100 nF capacitor from the output node to ground.

6. Add protection (optional) If connecting to a real MCU ADC, add a 1k series resistor at the very end, followed by two Schottky diodes: one from the signal to VCC (clamps high), one from GND to the signal (clamps low). This prevents the ADC input from exceeding the safe range.

7. Measure at room temperature Record the voltage at the thermistor divider output, the buffer output, the amplifier output, and the filter output. They should all be very close (since gain is approximately 1). Typical room temperature reading with 3.3V supply: about 1.65V.

8. Test with hot water Dip the thermistor in warm water (be careful with the wiring). The voltage should decrease (NTC thermistor: lower resistance when hot, lower voltage in the bottom position of the divider). Record the voltage.

9. Test with ice water Dip the thermistor in ice water. The voltage should increase. Record the voltage.

10. Calculate temperature from voltage Use the thermistor B-parameter equation to convert measured voltage back to temperature:

    • From the ADC voltage, calculate thermistor resistance:
    • From resistance, calculate temperature using the B-parameter equation

Verification Checklist

Connecting to a Microcontroller

If you have an Arduino, STM32 Blue Pill, or any MCU with an ADC:

1. Connect the filter output to an ADC input pin
2. Connect the circuit ground to the MCU ground
3. Read the ADC value in your firmware
4. Convert ADC count to voltage:
5. Convert voltage to resistance:
6. Convert resistance to temperature using the Steinhart-Hart or B-parameter equation
7. Print the temperature to a serial terminal

This is exactly the signal path used in production temperature measurement systems, from industrial process control to weather stations to HVAC systems. The principles are identical whether you are building on a breadboard or designing a multi-layer PCB.

Noise Reduction Techniques

Hardware Techniques

Software Techniques

The best approach combines both: hardware filtering removes noise before the ADC, and software filtering cleans up the remaining noise in the digital domain.

Common Mistakes

How This Connects to Embedded Systems

Course Summary

You have now covered the complete foundation of analog electronics for embedded systems:

These nine lessons give you the vocabulary and practical skills to understand, design, and debug the analog portions of any embedded system. When a circuit behaves unexpectedly, you now have the tools to measure, analyze, and fix the problem at the component level.