Thermistor Linearization Challenges

This article and video presentation by Sam Ben-Yaakov challenge conventional wisdom and reveal that the widely recommended method of adding a parallel resistor across the thermistor doesn’t actually enhance linearity when evaluated accurately. Instead, it diminishes sensitivity and creates a deceptive visual improvement in plots that aren’t normalized for span. 

Introduction

NTC thermistors are widely used for temperature sensing thanks to their high sensitivity, low cost and simple interface with standard analog circuits and ADCs.

Despite these advantages, the strongly non-linear resistance–temperature characteristic complicates accurate measurement when a simple voltage divider is used. A popular “analog linearization” technique is to add a fixed resistor either in series or in parallel with the thermistor in order to obtain a more linear voltage–temperature response over a limited range.

Core Concepts of Thermistor Linearization

NTC thermistor characteristics

Negative Temperature Coefficient (NTC) thermistors exhibit a resistance that decreases rapidly with increasing temperature, often modeled by an exponential or Steinhart–Hart-type relationship over the operating range:

For many power electronics and general sensing applications, the relevant temperature span is roughly from −40 °C to +150 °C, within which the resistance can change by an order of magnitude or more. The high sensitivity (large dR/dT) makes NTC thermistors attractive for ADC-based measurement, but the associated non-linearity means that a direct voltage divider output is far from linear in temperature.

Common analytical models

A frequently used simplified model for an NTC thermistor relates resistance R(T) at temperature T to a reference resistance R_ref at T_ref using an exponential law equivalent to a truncated Steinhart–Hart expression. Manufacturers often provide either explicit Steinhart–Hart coefficients or compact exponential fits, and many also supply ready-to-use SPICE macro-models that realize the non-linear resistance in simulation. Such models are accurate enough across the typical operating interval to evaluate linearization strategies quantitatively.

Classical Linearization Approaches

Series-resistor method

The simplest configuration uses the thermistor R_T in series with a fixed resistor R_S driven from a reference voltage, with the output taken across the thermistor (or across the resistor, depending on polarity preference). A widely taught rule of thumb states that for best “local linearity” around a chosen temperature T₀, the series resistor should be selected approximately equal to the thermistor resistance at that temperature, i.e. R_S ≈ R_T(T₀). This configuration yields a reasonably linear response only within a relatively narrow temperature band surrounding T₀, and becomes increasingly non-linear as the temperature deviates from that point.

Parallel-resistor “improvement” method

To extend the apparently linear region, many application notes propose adding a fixed resistor R_P in parallel with the thermistor, while still using a series resistor R_S to form the voltage divider. Published design recipes provide closed-form expressions or graphical methods to select R_S and R_P given a nominal resistance at a reference temperature and a target measurement range, and they typically present plots where the output curve with R_P appears visually closer to a straight line. This visual impression has encouraged the belief that the parallel resistor genuinely improves linearity over the same measurement span.

Revealing the Linearization Misconception

Voltage span versus linearity metric

A crucial observation is that many “before and after” plots compare a full-span response without the parallel resistor (for example, output varying from near 0 V to near the supply voltage) to a much narrower-span response when R_P is added. With the thermistor shunted, the effective resistance range shrinks and the output voltage swing across the temperature range of interest may be reduced from, say, 10 V to 3 V, which naturally compresses deviations from an ideal straight line when viewed on the same absolute vertical scale. This creates an illusion of improved linearity, even though the ratio of deviation to total span (and therefore the effective nonlinearity in V/°C) has not necessarily improved.

Using a normalized linearity criterion

To assess linearity meaningfully, the deviation from a best-fit straight line must be considered relative to the overall sensitivity, typically characterized as volts per degree Celsius, not just in absolute voltage units. When the output curves are scaled to the same span (for example, by dividing the “no parallel resistor” curve by an appropriate factor so that both responses cover the same vertical range), the apparent advantage of the parallel resistor disappears and the two curves exhibit essentially the same shape of non-linearity. In other words, the parallel resistor has merely reduced sensitivity rather than improving the intrinsic linear relationship between temperature and output voltage.

Thevenin-Based Circuit Interpretation

Equivalent circuit derivation

Consider the standard configuration with a supply voltage, a series resistor R_S, and the thermistor R_T(T) in parallel with a fixed resistor R_P, with the output taken at the junction. By “cutting” the circuit at the output node and replacing the source plus series resistor network with its Thevenin equivalent as seen from that node, one can show that the series–parallel arrangement is equivalent to a single effective series resistor plus a Thevenin voltage source. The parallel network of R_S and R_P can be collapsed into one resistance, and the voltage division between them yields a fixed Thevenin source that subtracts from the original supply as seen by the thermistor.

Impact on effective excitation and sensitivity

In this Thevenin view, the thermistor is still driven by a series resistance, but now the effective excitation voltage is reduced because part of the supply is dropped across the internal Thevenin source in the opposite direction. Since the linearity of the voltage–temperature transfer is governed by the interaction between the thermistor’s non-linear resistance and the single series resistance, the presence of a fixed offset source does not change the non-linear functional shape; it only shifts and scales the response. As a result, the circuit with a parallel resistor yields essentially the same normalized non-linearity as a single appropriately chosen series resistor, but with lower overall sensitivity (smaller volts-per-degree span).

Simulation-Based Evidence

LTspice test setup

To quantify these effects, a non-linear resistor model representing the NTC thermistor is implemented in LTspice using an expression that encodes the exponential resistance–temperature relationship, fitted to manufacturer data at a reference resistance R₂₅ (for example, 10 kΩ at 25 °C). A transient simulation sweeps a “temperature” control variable from −40 °C to +150 °C using a ramp source, and the output voltage across the thermistor is recorded together with its temperature derivative to obtain sensitivity in volts per degree (with optional scaling for convenient plotting). Multiple subcircuits are instantiated: one with only a series resistor, one with an additional parallel resistor of a value borrowed from a published example, the corresponding Thevenin-equivalent circuit, and a variant where the Thevenin source is removed but the equivalent resistance is kept.

Results and interpretation

The simulation shows that the “series plus parallel” and its exact Thevenin-equivalent circuit produce identical output and sensitivity curves, as expected from linear circuit theory. When comparing the series-only configuration to the series-plus-parallel configuration, the latter exhibits a much smaller output span and significantly reduced sensitivity, even though its curve may look slightly “flatter” on an absolute scale. After scaling the high-span series-only response down by the same factor by which the Thevenin offset reduces the effective voltage, the normalized curves overlap closely, demonstrating that the parallel resistor has not improved the intrinsic linearity, only sacrificed sensitivity.

Practical Design Implications

Why the parallel resistor method persists

The popularity of the parallel resistor method likely stems from design flows where the initial choice of series resistor is suboptimal; adding a parallel resistor then “trims” the effective resistance seen by the thermistor and incidentally brings the operating point closer to an optimum. Because many published plots omit normalization and show only absolute voltages, this trimming effect can be misinterpreted as a fundamental linearization improvement attributable to the parallel branch itself. In reality, an equivalent or better result could be achieved by simply adjusting the series resistor alone, without the undesirable reduction in sensitivity and without introducing a DC offset through the Thevenin source.

Recommended analog front-end strategy

For analog front-ends that still rely on a simple divider, the preferred strategy is to:
1) Select a single series resistor that optimizes linearity and sensitivity over the temperature interval of interest, using simulation or numerical optimization rather than only a rule of thumb.
2) Avoid adding a parallel resistor solely for the sake of linearization, since it does not improve normalized linearity and instead diminishes the useful signal amplitude. If a reduced span is needed to match ADC range or protect an input, an explicit attenuator following the optimized divider is more transparent and does not entangle span control with linearity.

Alternative Approaches to Linearity

Digital linearization

In modern designs, the most robust and accurate way to handle thermistor non-linearity is to digitize the divider voltage directly and apply either the full Steinhart–Hart equation or a high-resolution lookup table in firmware. With adequate ADC resolution, calibration data and computational resources, this approach can linearize the response to within fractions of a degree over wide temperature spans, making purely analog linearization networks unnecessary except in legacy or ultra-low-cost designs.

Alternative analog techniques

Where digital post-processing is not feasible, more sophisticated analog approaches—such as using complementary non-linear elements, transistor-based shaping networks or multi-segment piecewise-linear approximations—can be employed to better match the thermistor’s exponential behavior. However, these solutions add complexity and component count, and for most practical systems the combination of a well-optimized simple divider and digital correction provides a superior trade-off between accuracy, flexibility and cost.

Illustrative Design Data

Example parameter table

Table 1 summarizes an illustrative comparison between a “series-only” and a “series plus parallel” configuration over a nominal −40 °C to +150 °C range, based on the qualitative trends reported in the reference analysis. Exact numeric values depend on the particular thermistor and resistor choices, but the relative trends are generic.

Metric	Series-only divider	Series + parallel resistor
Output span over −40 °C to +150 °C	Large span (e.g. ~0 V to near supply)	Much smaller span (e.g. ~30–40 % of supply)[web:1]
Sensitivity (V/°C)	High sensitivity due to large span	Significantly reduced sensitivity[web:1]
Apparent deviation from straight line (absolute volts)	Visibly larger on unnormalized plots	Visibly smaller, but largely due to compressed scale[web:1]
Normalized deviation (relative to span)	Comparable to parallel case after scaling	Comparable to series-only after proper normalization[web:1]
DC offset / shift of operating point	None beyond divider behavior	Introduces offset via Thevenin source equivalent[web:1]
Component count	One resistor + thermistor	Two resistors + thermistor

Conclusion

The analysis of thermistor linearization networks shows that adding a fixed parallel resistor across an NTC thermistor in a divider does not intrinsically improve linearity when performance is judged using a normalized metric such as deviation relative to volts-per-degree sensitivity.

Interpreted through Thevenin equivalence and confirmed in LTspice simulations, the series-plus-parallel configuration is effectively equivalent to a single series resistor driven from a reduced excitation voltage, leading primarily to a loss of sensitivity rather than an improvement in transfer linearity.

For analog thermistor front-ends, designers are therefore encouraged to optimize a single series resistor (optionally followed by simple attenuation if necessary) and, where possible, to rely on digital linearization—via Steinhart–Hart or lookup tables—to achieve high accuracy over wide temperature ranges.

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