NTC Design Calculator

The NTC Design Calculator is an advanced tool that helps engineers design temperature-dependent voltage sensing and biasing networks using an NTC thermistor and an operational amplifier. It calculates the ideal resistor values (R2, R3, R4) for a given supply voltage, desired temperature range, and output voltage span, ensuring accurate linearization across the sensing range. The calculator also determines Vref, gain, and output limits, supports E24/E96 rounding for resistor selection, and includes an optional input RC filter for noise reduction. This makes it an ideal utility for analog front-end design, temperature-to-voltage conversion, and thermal feedback networks in motor drives, power converters, and embedded systems.

NTC Circuit Formulas and Theory

An NTC thermistor is a temperature-dependent resistor whose resistance decreases exponentially with temperature. The relationship follows the Steinhart–Hart or simplified Beta equation: R(T) = R25 × exp[β × (1/T − 1/298.15)], where R25 is the resistance at 25 °C and β is the material constant in kelvin. In this circuit, the NTC forms a voltage divider with a fixed resistor, creating a temperature-sensitive voltage that is fed into an op-amp configured for biasing and gain adjustment. The output voltage varies linearly across the temperature range as VOUT = VREF + G × (VNTC − VREF), where VREF is set by the R2–R3 divider and G is defined by R4 feedback. This configuration enables precise analog temperature measurement and conditioning for ADC or control inputs.

NTC thermistor divider with R2, R3 bias network and op-amp feedback resistor R4 providing gain.

NTC + Op-Amp Signal Conditioning Calculator

NTC Divider
RNTC @ Tmin (ohm)
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RNTC @ Tmax (ohm)
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R1 (optimal) (ohm)
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R1 (selected) (ohm)
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Vin @ Tmin (V)
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Vin @ Tmax (V)
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Ideal Gain
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(R2 // R3) ideal (ohm)
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Bias + Gain Network
Vref target (V)
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R2 ideal (ohm)
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R3 ideal (ohm)
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(R2 // R3) actual (ohm)
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R2 selected (ohm)
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R3 selected (ohm)
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Vref actual (V)
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Gain actual
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Vout @ Tmin (V)
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Vout @ Tmax (V)
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Mapping Error (min/max)
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Sanity
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Input RC Filter
Cf (F)
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Cf (uF)
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Time constant τ (ms)
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Settling ~5τ (ms)
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Self-tests
(No tests run yet)

How to Use the NTC Design Calculator

This calculator helps you design NTC-based temperature sensing and bias networks using an operational amplifier. Enter thermistor parameters, temperature limits, and desired output voltage range to generate the complete resistor network configuration.

1) Input NTC Data

  • Enter the thermistor’s R25 and β (Beta) value from its datasheet.
  • The calculator uses this to determine resistance variation with temperature.
Example: R25 = 10 kΩ, β = 3950 K.

2) Set Temperature Range

  • Specify Tmin and Tmax for your application.
  • These define the operating window and determine the corresponding NTC resistance values.

3) Define Output Targets

  • Enter VDD (supply) and target output voltages at Tmin and Tmax.
  • The tool computes the required gain and Vref for linear mapping.

4) Compute Bias Network

  • The calculator determines R2 and R3 for the bias divider, and R4 for op-amp feedback gain.
  • It supports E24 / E96 rounding and lets you test alternate resistor sets.

5) Review Results

  • Check calculated values for Vref, gain, and Vout range.
  • The tool highlights mismatches or clipping if outputs exceed supply limits.

6) Optional RC Filter

  • Set a target cutoff frequency and input resistance to compute the filter capacitor (Cf).
  • The calculator reports τ and 5τ response time for stability and noise filtering.

Quick Checklist

  • Enter accurate NTC β and R25 values
  • Confirm Tmin and Tmax are realistic for your environment
  • Set output range within ADC input or signal conditioning limits
  • Use precision resistors (≤1%) for better linearity
  • Validate results after rounding to E-series parts
  • Add RC filtering to suppress ADC noise
FAQ & Tips

What if my output doesn’t match targets?
Adjust R2/R3 ratio slightly or change R4 gain to better align with expected output range.

Why use E24 or E96 rounding?
Real resistors come in standard values; rounding ensures components are practically available.

Can I use this for digital sensors?
This calculator is designed for analog NTC front-ends. For digital sensors (I²C, SPI), no bias network is required.

What resistor types are best?
Use metal film or thin-film resistors for low drift and high accuracy in temperature-critical circuits.

Copy-Paste Mini Workflow

1) Enter R25 and Beta from the NTC datasheet
2) Set Tmin and Tmax for your target sensing range
3) Define supply (VDD) and desired output range
4) Compute network → note R2, R3, R4 and output voltages
5) Round to E24/E96 and verify results
6) Add RC filter for stable, noise-free readings