There is growing demand for sensors in applications including industrial automation, IoT, automotive and medical devices. In sensor circuits, resistors are used in voltage dividers to scale voltages, in feedback networks to provide compensation and in signal conditioning circuits for filtering, damping and protection.
Resistor imperfections contribute to measurement errors, noise, and/or drift. Since the perfect resistor does not exist, it is essential to be aware of resistor limitations and select the appropriate component for the application. Invariably, there are trade-offs between cost and performance to consider.
In this article, we consider the demands of sensor applications and compare common resistor technologies.
Sensor Application Demands
The challenges sensor designers face in industrial and IoT applications include:
- Fundamentals – accuracy, precision, linearity, resolution, dynamic range, response times.
- Calibration – at factory and re-calibration in the field.
- Drift and aging.
- Diagnostics and remote monitoring.
- EMI/RFI.
- Noise and signal integrity.
- Grounding, shielding and isolation.
- Reference stability.
- Power management.
- Environmental issues – Thermal cycling, mechanical stress, humidity, chemicals, dust.
- Interfaces & cabling limits.
- Space constraints.
- Compliance, security and safety issues.
Appropriate resistor selection can have a positive impact on many of the above challenges. A poor selection can do the opposite.
Comparing Resistor Technologies
Power Rating
In many sensor elements, power consumption is very low (often µW to low mW). However, overall system power is frequently dominated by supporting electronics such as signal conditioning (amplifiers and references), high-performance ADCs, local signal/data processing, and wireless data transmission. Some sensor types, particularly those with heaters or active emitters, can draw tens to hundreds of milliwatts while operating.
Low power operation means most resistor technologies, including wirewound, thin film, thick film and metal foil are suitable for sensor applications. Once power starts to rise, then thick film or wirewound resistors dominate.
Accuracy / Precision / Drift / Noise
If accuracy and precision are primary concerns, then thin film or metal foil resistors tend to be the best choice, especially if limiting noise and drift at very low signal levels is an issue.
Thin film devices are widely available with tight tolerance values delivering low drift, low noise and VCR. Thin film resistors are widely used in uncalibrated systems, precision gain-setting, bridges and in low-noise sensor front-ends.
Metal foil resistors excel in sensor applications where excellent accuracy, precision and stability are required, as they deliver extremely low drift and have excellent temperature behaviour. The trade-off is cost and availability, hence Metal foil resistors are typically found in high-end bridge instrumentation, precision references/dividers, and in long-life systems where recalibration is difficult.
Stability
Stability is defined by how much a resistor value (or resistor ratio) changes over time. Key factors that impact stability are temperature coefficient of resistance (TCR), humidity and contamination, mechanical stress, thermoelectric effects, aging, drift and noise.
Assuming potential humidity and contamination issues are addressed, Metal foil resistors are the most stable in sensor applications, followed by thin film. Where a more robust solution is required, thick film resistors or wirewound resistors are preferred.
Wirewound resistors are typically found in higher-power sensor applications, particularly in harsh environments, but the resistors’ high inductance can be a limiting factor. Thick film is rarely the best choice where stability is critical, but it’s often the best engineering choice when robustness is an issue, the sensor system is ratiometric, calibratable or bandwidth-limited, and cost/availability matter.
Voltage Co-efficient Of Resistance (VCR)
If a resistor’s value depends on the voltage across it, then changes in operating voltage can translate into errors. Resistors with a relatively high voltage coefficient of resistance (VCR) can introduce subtle nonlinearity or mode-dependent drift, especially when voltages vary.
Metal foil and thin film resistors have low VCR. Wirewound resistors also have excellent VCR but relatively high inductance, component size and thermal issues limit their applications.
In general, Thick film resistors have the widest VCR spread, especially at high resistance, but the technology remains a strong sensor choice when the voltage across the resistor is small, ratios/cancellation are used, or calibration is available. Specialist thick film resistor manufacturers can optimise resistor VCR, but this can be at the expense of other parameters.
Mechanical & Thermal Stress
Metal foil resistors offer excellent stability under both mechanical and thermal stress. While thin film resistors also perform well under thermal stress, they are more susceptible to mechanical stress issues.
Thick film and wirewound resistors are the most thermally and mechanically robust, but do not perform as well as thin film or metal foil in low-power sensor applications (see above). Wirewound devices are large and have high inductance.
Humidity, Chemicals & Dust
The performance of all resistor technologies in high-humidity, corrosive chemical and dust environments depends on the quality of encapsulation/passivation and suitable termination plating.
Good coating and plating greatly reduce humidity and chemical problems, but they don’t remove all differences between resistor types, especially at high resistance values. Here, any tiny surface/volume leakage or contamination sensitivity becomes important.
Space Constraints
For space-constrained applications, thick film resistor technology offers the most compact solution, with thin film and metal foil resistors also providing relatively small footprints. Wirewound resistor devices are the largest option for any given power rating.
The primary factor determining resistor size is its power rating. When high power handling is required, thick film and wirewound resistors excel, offering superior capabilities compared to thin film or metal foil alternatives. Thick-film power resistors can achieve high power dissipation in a compact form factor, but heat management is critical.
Cost/Performance Trade-offs
Thick film resistors generally win when considering cost vs performance, offering acceptable performance at a relatively low price. Specialist thick film devices can be optimised to compete well with other resistor technologies on specific parameters, but this tends to increase the price.
Wirewound resistors are relatively low cost and are used in sensor applications where size and inductance is not an issue. If sensor accuracy is a primary concern, then it can be worth paying more for thin film. For specialist applications, metal foil resistors offer excellent accuracy, precision and stability at a premium price.
Customization
Thick film resistor technology delivers design flexibility. Specific parameters can be fine-tuned to match the demands of specialist applications. As mentioned above, the technology has a small form factor, but it can also be shaped to fit where necessary. Thin film resistors can match the customisation of thick film to an extent, but not at scale and only at significantly higher cost.
Conclusion
Where accuracy and precision are the top priorities, metal oxide, carbon film, and carbon composition resistors are generally avoided because they produce more noise and tend to drift more with humidity, age, and mechanical or thermal stress.
Thin film resistor devices offer tight tolerance values and deliver low drift, low noise and VCR, but have limited power handling capability and are relatively high cost. Metal foil devices deliver excellent stability and low drift, low noise and VCR but are niche and cost-prohibitive. Wirewound devices are appropriate where low VCR is a concern, but are bulky and inductive.
Thick film resistors are robust, customizable and relatively low-cost. They are ideal when performance and practicality matter, delivering a balance of cost, reliability, and flexibility. The technology continues to evolve with new materials and manufacturing advances.
While metal foil resistors can achieve sub-ppm stability, they’re often unnecessary for most sensor circuits where ±1% accuracy and long-term stability over temperature are sufficient. While resistor tolerance is vital in some sensor applications, ratio matching in dividers/bridges often matters more than absolute value.
When choosing resistors for sensor applications, it’s important to carefully consider the application and strike a balance between performance, availability, flexibility and cost.