For high performance applications a network resistor with low TCR, close ratio tolerances and long term in circuit stability is required. In this post, we consider the key network resistor specification issues a system designer should consider. Temperature effects, reliability, stability and drift and resistor matching all need careful consideration.
The Advantages Of A Network Resistor
The key advantage of resistor networks is the ability to combine several resistors in the same, high density package. Resistor networks are often constructed in single in line package. This increases the packaging density and improves cooling.
Compared to discrete devices a resistor network delivers increased reliability, improved temperature performance, improved stability and precision resistor matching.
Resistor Network Applications
Network resistors are found in a range of applications including resistor divider (precision amplifier) circuits, measurement systems, test equipment, industrial automation and pull up / pull down resistor arrays.
The simplest resistor network configuration consists of many resistors of the same value connected to a common bus. They tend to be used in pull up / pull down resistors, impedance matching and terminating applications.
For ECL terminations a network consisting of several resistors of various values connected in pairs to a common bus may be manufactured. The resistor values in each pair are chosen to form standard Thevenin equivalent resistance values for termination of two different voltages.
Where resistors of the same value are isolated from each other the networks are used in current limiting and termination applications. They are also occasionally used in systems where a number of same value resistors are connected to various points in a localised system board area.
One of the major disadvantages of discrete resistors is variations in Temperature Coefficient of Resistance. With different TCR’s the same change in temperature can result in a different change in resistance in two discrete devices. Also, if two discrete resistors are not in close physical contact external heat sources may impact on one resistor more than the other.
Appropriate choice of materials and manufacturing methods can minimise variations in TCR across two (or more) resistors in the resistor array. With resistor devices in close proximity differential temperature effects are reduced.
Network Resistor Reliability
At the system level a network resistor can improve reliability as on-board interconnect is reduced. Instead, interconnect between resistors is routed on the network resistor substrate.
At the resistor component level failures are generally due to environmental factors such as mechanical and electrical stresses.
The choice of resistor substrate material is directly linked to thermal performance. It also affects performance under pulse or surge conditions. The substrate design impacts on its mechanical performance, particularly when vibration or other mechanical stresses are present.
Various substrate materials are available but for many applications thick film technology is the preferred choice. The technology offers high packing density, superior mechanical properties, and excellent thermal performance.
The granular composition of the thick film resistor material can make it susceptible to thermal and electrical stresses. This particle to particle contact can be disrupted by various stress factors. This tends to cause a long-term change in performance rather than a complete failure.
However, correct selection of materials and appropriate manufacturing processes can minimise the potential impact of mechanical and thermal stress.
Stability And Drift
One of the key advantages of a network resistor is the long-term stability of resistor values (in relation to each other). By manufacturing multiple resistors at the same time, on the same substrate, using the same materials and process long term stability is maximised while the impact of external factors is minimised.
Matched resistor devices are typically used in precision amplifier resistor divider applications. A mismatch in the performance of the resistors in the divider must be avoided. While resistor tolerance matching, load life stability and matched temperature performance are all important considerations.
In an application specific resistor network, there may be several resistors each with their own value and tolerance. The manufacturing challenges discussed above are therefore compounded. The choice of materials and manufacturing methods is of critical importance in these applications.
Close co-operation is required between the resistor network manufacturer and the system designer to produce a component that meets the specification and will continue to perform over its required in service life.