Discrete thick-film resistors are the first choice in many applications, but their accuracy can be compromised by environmental effects particularly where the ratio between resistances matters. Temperature fluctuations and mechanical stress can introduce errors, leading system designers to seek alternatives.
One potential solution is the thick-film resistor network: a single ceramic substrate that integrates multiple resistive elements, conductor tracks, and terminations. This article examines environmental issues affecting discrete resistors and considers when it makes sense to consolidate multiple resistors into a resistor network.
Resistor Network Applications
Thick Film Resistor networks are found in a wide variety of applications including:
- Test, measurement and sensing.
- Power and Industrial Controls.
- Automotive and defence.
The reasons for choosing a resistor network in each application can vary; however, common issues across all applications are resistor matching, space saving, cost and reducing component count (hence, increasing reliability).
Test Measurement & Sensing Applications
Thick film resistors are often found in differential amplifier and precision circuits, sensors and sensor interfaces, and bridge circuits, where their stability, reliability, and cost-effectiveness make them a practical choice across many applications.
They are used in divider networks, gain-setting circuits, and ratio-matching applications, where it’s the ratio between resistors, not their absolute values, that defines performance.
Poor resistor matching can change the intended resistance ratio, which results in voltage-divider, gain, bias, threshold, and calibration errors. In differential or bridge circuits, it can also reduce common-mode rejection and introduce offset, noise sensitivity, and cause channel-to-channel variation.
A circuit can become inaccurate over time due to differences in temperature coefficient of resistance and self-heating.
Power and Industrial Control Circuits
In power supplies, discrete thick-film resistors are used as bleeder and discharge resistors, as voltage-balancing resistors across series-connected capacitors, in snubber and damping networks, in start-up and bias circuits, and in output-voltage sensing, feedback and control-loop compensation circuits.
Resistor matching is most important where circuit performance depends on a resistance ratio or equal current sharing. For example In output-feedback and line-voltage sensing circuits, a resistor-ratio mismatch can alter the sensed voltage, regulated output and measurement accuracy. In differential and bridge current-sensing circuits, a mismatch can also degrade common-mode rejection and introduce gain and offset errors.
A mismatch in voltage-balancing resistors can cause unequal voltage distribution across series-connected capacitors, rectifiers or switching devices. In current-sharing networks, it can lead to unequal current distribution between parallel power stages or devices.
Automotive and Defence Applications
In automotive and defence applications, in addition to the issues outlined above, reliability is a key issue. Solder joints can crack if subjected to repeated thermal cycling and/or vibration. Mechanical shock is also a potential issue.
The Causes of Resistor Mismatch
It’s clear that mismatches between the actual and expected resistance values (or resistance ratios) of two or more discrete resistors can impact system accuracy, gain, offset, linearity and stability.
A pair of discrete 1% resistors could differ by nearly 2% in the worst case. However, Initial tolerance is not the only source of mismatch, as the resistance ratio may also change with temperature.
Over a limited temperature range, a resistor’s change in resistance is approximately proportional to its TCR and the change in its body temperature. Due to manufacturing variations individual resistors often have different TCR values even when resistors share the same datasheet TCR specification.
In addition to these temperature effects, discrete resistor values can drift over time. Two discrete resistors may have slightly different material composition, manufacturing process (including trimming) and residual mechanical stress. Exposure to temperature, humidity and electrical loading can cause them to exhibit differences in long-term resistance drift.
Discrete resistors can also introduce issues unrelated to the resistance values. PCB trace lengths between discrete resistors are rarely identical, so stray capacitance and inductance differ slightly from resistor to resistor.
How a Resistor Network Helps
A resistor network reduces temperature-related ratio errors because its resistor elements are manufactured together on the same substrate. They are therefore subjected to the same processing, mechanical stress, and temperature conditions. Their absolute resistance values can still drift with temperature, but as their TCRs are closely matched, they tend to drift by the same percentage, hence preserving their ratio.
The network construction also increases long-term stability. As elements are fabricated together, a matched resistor network experiences less relative ageing, delivering improved long-term ratio tracking than a set of discrete resistors.
Integration delivers an additional advantage beyond resistance matching and stability: it also reduces differences in electrical characteristics. Since the internal layout of a network is fixed and symmetrical, parasitic mismatch is much lower than multiple discrete resistors on a system board.
Finally, shared construction improves system-level reliability. Years of thermal and mechanical stress can degrade solder joints. As a resistor network requires fewer joints, it reduces this potential failure mode and improves overall system reliability.
The above advantages combined with the smaller footprint of a single package compared with several discrete resistors, make resistor networks useful in sensor modules, instrumentation, and high-density assemblies where space is limited.
Cost Comparisons
A resistor network will always be more expensive than the equivalent discrete resistors, particularly in low volume. A custom network only makes sense when resistor mismatch and ageing have an unacceptable impact on performance or when reliability, total assembly cost and board space are key issues.
Discrete resistors remain the best choice for low-volume and simple circuits, early prototyping and where frequent circuit updates, including resistance value changes, are expected. Custom thick film resistor networks can be designed to deliver specific functions in a single package.