The purpose of a custom resistor network is to solve system design challenges.
Applications include resistor divider circuits, resistor ladders, measurement systems, test equipment, industrial automation and pull up/pull-down resistor arrays.
The ideal resistor network will have low TCR, close-ratio tolerances and long-term stability. To deliver a high-performance product the resistor manufacturer must consider many design issues. They include:
● Resistor values and tolerances.
● Resistor matching.
● Power rating.
● Surge and transient conditions.
● Stability and drift.
● Temperature effects.
● Material selection and reliability.
● Operating voltage
Some applications need a network of resistors of varying values on the same network resistor substrate. The design and manufacturing process for these devices can be complex. Particularly if there is a wide variance in resistor values.
Different resistor values may require the use of different materials and varying amounts of substrate area. Design compromises are inevitable.
Resistor Matching In A Network
Matched resistor devices are typically used in precision amplifier resistor divider circuits. In these applications matched resistor tolerance and temperature performance are important considerations. Load life stability is also an issue.
The choice of materials and manufacturing methods is critical. This is particularly important if there are several resistors each with their own value and tolerance in the network.
Custom Resistor Network Power Rating
It is important to establish the normal operating power rating of each resistor device in the network.
The sum of these values gives the total power rating. This value, combined with the
connection details and substrate material specification, determines the resistor network dimensions. Often, final dimensions are then established using load life tests.
Application issues can force resistor network design modifications. These include ambient temperature, cooling and the proximity of other heat-generating components.
Sometimes it is necessary to derate the resistor network device in the application.
Surge And Transient Conditions
Surge or transient conditions can reduce the operational life of a resistor network. Or, in some cases, cause catastrophic failure. It is important to understand these conditions and design the custom resistor network accordingly.
The mass of the network, the geometry of the resistor and the final resistor trim all influence surge survivability. The design and manufacturing issues are more complex when several different resistor values are in the same network.
Resistor 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).
To maximise long term stability and minimise the impact of external factors, multiple resistors are manufactured at the same time, on the same substrate, using the same materials and process.
Temperature Effects In A Resistor Network
A major disadvantage of discrete resistors is variations in the Temperature Coefficient of Resistance (TCR). With different TCR’s the same change i n temperature can result in a different change in resistance in two discrete devices.
If two discrete resistors are not in
close physical contact external heat sources may impact one resistor more than the other.
Material selection and manufacturing methods can minimise TCR variations across resistors in the network array. The close proximity of resistor devices reduces differential temperature effects
Material Selection And Reliability
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 can cause a long-term change in resistor performance. Or, in extreme cases
complete resistor failure.
The choice of resistor substrate material is directly linked to thermal performance. It also affects performance under pulse or surge conditions and resistor mechanical performance, particularly when vibration or other mechanical stresses are present. However, material selection and optimised manufacturing processes can minimise their impact.
Custom Resistor Network Application Example
When a basic digital to analogue conversion is required a resistor ladder is a relatively low-cost option.
As the ladder operates as an array of voltage dividers, resistors within the ladder are matched (as close as practical). Ideally, the voltage ratio for a given bit is precisely half that of the preceding bit.
Variations in resistor tolerance across the array, the temperature coefficient of resistance (TCR) and drift in resistance across the array all impact on output accuracy. Resistor networks designed and manufactured by specialist thick film resistor manufacturers can resolve these issues.
Thick film resistor networks are mechanically robust. Resistors can be closely matched to eliminate tolerance issues. As resistor devices in a resistor network are in close proximity on the same substrate the impact of temperature and variations in resistor life is minimised.
Custom resistor networks are often required in low to medium volume. This can mean they are not of interest to the major resistor manufacturers. However, specialist resistor network manufacturers focused on the application-specific marketplace can often help.