Resistor networks have several advantages over discrete devices. They reduce the system board area required to mount resistor devices, increase system reliability (lower number of interconnects) and simplify the system manufacturing process.
Resistor networks are used extensively in simple applications such as pull up/pull down resistors and terminating resistors. In these applications resistor tolerance and the impact of a change in one resistor value in relation to another is not important.
In more specialist applications custom resistor networks are used to solve system design challenges including thermal and reliability issues, resistor and tolerance matching and TCR matching.
Film technology (thick and thin film) is used to manufacture the majority of resistor network devices. Thin film technology tends to be used in precision, low power applications with thick film reserved for higher power and higher resistor value applications. In applications where transient overvoltage and current are an issue, thick film is the preferred choice.
When designing a custom resistor network there are several issues to consider:
- Resistor values and tolerances
- Power rating.
- Surge and transient conditions.
- Temperature coefficient of resistance (TCR).
- Dimensions and connection method.
Resistor Values And Tolerances
In some specialist applications, a network of several resistors of varying values on the same network resistor substrate may be required. 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. Close co-operation between the system designer and the resistor manufacturer is required to agree on the inevitable design compromises.
It is important to first establish the normal operating power rating of each device and sum these to give the total power rating. This, coupled with the connection details, will give a first indication of the resistor network dimensions.
Surge And Transient Conditions
Surge or transient conditions can reduce the operational life of a resistor network or cause catastrophic failure. It is important to understand these conditions so the resistors can be designed accordingly.
The mass of the network, the geometry of the resistor and the final resistor trim all influence the resistor network design and manufacturing process. Designing for surge survivability is more complex when several different resistor values are used in the same network (see above).
Temperature Coefficient of Resistance (TCR)
In most applications matching each of resistor devices Temperature Coefficient of Resistance (TCR) is crucial to minimise the impact of changes in ambient temperature.
The system designer must clearly state the expected change in the value of a single resistor in response to a change in temperature (TCR). Perhaps, more important, they must also be able to specify the TCR tracking. That is the expected change in the ratio of two or more resistors in response to a change in temperature.
Custom resistor networks tend to be required in low to medium volume which often means they are not of interest to the major manufacturers. However, specialist resistor manufacturers focused on the application specific marketplace can often help. Delivery of a successful end product requires close co-operation between the system designer and the manufacturer.