Sensors & Measurement · Study deck

Resistive Dividers and ADC Loading

Picture a water sensor whose resistance changes as a tray fills.

Physics Phoebe is your guide for this deck.

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

Learning objectives

You will be able to:

  • Explain how thermistors, LDRs, FSRs, and potentiometers convert physical changes into resistance changes.
  • Use voltage-divider math to convert resistance into an ADC-readable voltage.
  • Apply the ratiometric trick so supply drift cancels from ADC codes.
  • Choose divider values that balance sensitivity, battery current, and ADC source-impedance limits.
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Major section

Start With the Measurement Story

The controller cannot read resistance by itself.

  • A pair of resistors turns the change into a voltage, and the input circuit turns that voltage into a number.
  • Each step can change the result.
  • A voltage divider means two resistors that share a supply voltage.
  • Firmware means software stored on the controller.

Key terms

Resolution
Resolution means the smallest change the number can show.
Loading
Loading means the input circuit changes the voltage it is trying to measure.
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Major section

Start With the Measurement Story (continued)

An analog-to-digital converter means a circuit that turns a voltage into a number; it is called an ADC.

  • Resolution means the smallest change the number can show.
  • Loading means the input circuit changes the voltage it is trying to measure.
  • This runway does not choose the best resistor or prove the sensor is linear.
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Major section

Divider Sensitivity and ADC Load

The divider calculation is incomplete until the ADC has acquired the node voltage.

  • Inside the: ADC,: Vref sets scale and: N bits set steps, but the third label—sample window—is the one threatened by a large Thevenin resistance: the sampling capacitor may not settle to Vin before conversion.

Key terms

Vin
Vin is the conditioned voltage presented by the divider.
Code
Code is therefore a digital count of the voltage actually acquired, as the displayed count = Vin / Vref × (2^N − 1) relation makes explicit.
Sensor input voltage Vin feeding an ADC referenced to Vref, where bit depth and sample time determine the resulting digital count code.
Sensor input voltage Vin feeding an ADC referenced to Vref, where bit depth and sample time determine the resulting digital count code.
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Major section

Turn a Warm Thermistor Into a Code

In the resistive divider, a thermistor on a pipe changes resistance, but the ADC pin can only see voltage.

  • A 12-bit ADC then reports about 1.65/3.3 × 4095=2048.
  • The direction of that 683-code change is part of the circuit contract.
Sensor input voltage Vin feeding an ADC referenced to Vref, where bit depth and sample time determine the resulting digital count code.
Sensor input voltage Vin feeding an ADC referenced to Vref, where bit depth and sample time determine the resulting digital count code.
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Deck summary

Key takeaways

The controller cannot read resistance by itself.

  • An analog-to-digital converter means a circuit that turns a voltage into a number; it is called an ADC.
  • The divider calculation is incomplete until the ADC has acquired the node voltage.
  • In the resistive divider, a thermistor on a pipe changes resistance, but the ADC pin can only see voltage.
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Retrieval practice

Recall check 1 of 3

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

Q1Why does a resistive sensor such as a thermistor or LDR usually need a voltage-divider circuit before it can be read by a microcontroller?

AThe sensor changes resistance, but the ADC can only measure voltage.
BThe divider amplifies the sensor's output power so the ADC receives enough current.
CThe divider removes all noise from the sensor signal.
DMicrocontrollers can read resistance directly, so the divider is purely optional decoration.
Show answer

Answer: A The divider is the bridge from a resistance the ADC cannot read to a voltage it can.

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Retrieval practice

Recall check 2 of 3

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

Q2A divider uses a fixed top resistor and a sensor bottom resistor, fed from the same 3.3 V rail that supplies the ADC reference. The 3.3 V rail sags to 3.1 V under load. What happens to the ADC code for a fixed sensor resistance?

AThe code is essentially unchanged
BThe code drops in proportion to the supply, because Vout scales with Vs.
CThe code doubles, because lower supply means higher resolution.
DThe sensor resistance changes to compensate automatically.
Show answer

Answer: A Feeding the divider and the ADC reference from the same rail makes the reading immune to supply variation.

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Retrieval practice

Recall check 3 of 3

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

Q3A thermistor divider built with 220 kΩ legs to save battery current reads consistently lower than a bench multimeter across the same nodes. What is the most likely cause?

AADC loading from high source impedance
BThe thermistor became more accurate, so the multimeter is wrong.
CHigher resistance always increases the supply voltage seen by the divider.
DThe ratiometric cancellation failed because the legs are large.
Show answer

Answer: A High-impedance dividers under-charge the S/H cap; lengthen the sample time, buffer the node, or lower the impedance.

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Print reference

Answers

Answer key.

  1. A · The divider is the bridge from a resistance the ADC cannot read to a voltage it can.
  2. A · Feeding the divider and the ADC reference from the same rail makes the reading immune to supply variation.
  3. A · High-impedance dividers under-charge the S/H cap; lengthen the sample time, buffer the node, or lower the impedance.
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