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Custom resistor networks address system design challenges. Applications include resistor divider circuits, resistor ladders, measurement systems, test equipment, industrial automation and pull-up/pull-down resistor arrays.

Key characteristics to consider in an ideal resistor network include a low-temperature Coefficient of Resistance (TCR), tight ratio tolerances, and robust long-term stability.

To deliver a high-performance product resistor, manufacturers must consider many design issues. They include:

● Resistor values and tolerances.
● Resistor matching.
● Power rating.
● Surge and transient conditions.
● Stability and drift.
● Temperature effects.
● Operating voltage

We consider each of these issues in more detail below.

Resistor Values & Tolerances

Some applications use resistors of varying values on the same network resistor substrate. If there is a wide variance in resistor values the design and manufacturing process can be complex.

Different resistor values may require different materials and varying amounts of substrate area. Design compromises are inevitable.

Matching in precision resistor networks requires precise control of the manufacturing process. The quality of the substrate material and its surface finish are important factors to consider.

Resistor Matching In A Network

Matched resistor devices are 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. By summing these values, we obtain the total power rating.

The properties of the substrate material, the power rating and interconnect limitations determine the dimensions of the resistor network. Final dimensions are often fine-tuned through load life tests.

Various application factors impact the resistor network design. These include ambient temperature, cooling conditions, and nearby heat-generating components. It is sometimes necessary to derate the resistor network device to ensure optimal performance.

Surge And Transient Conditions

Surge or transient conditions can reduce the operational life of a resistor network or, in some cases, cause catastrophic failure.

The mass of the network, the geometry of the resistor and the final resistor trim all influence surge survivability.

Resistor Stability And Drift

One of the key advantages of a network resistor is the long-term stability of resistor values (relative to each other).

To maximise long-term stability multiple resistors are manufactured at the same time, on the same substrate, using the same materials and process.

The close proximity of resistors and a controlled manufacturing process deliver better thermal coupling between resistors. This helps equalise temperature gradients across the network, reducing thermal-induced drift effects.

Temperature Effects In A Resistor Network

A major disadvantage of discrete resistors is variations in the Temperature Coefficient of Resistance (TCR). With different TCRs a change in temperature can result in a different change in resistance in two discrete devices. With discrete resistors, heat sources may impact one resistor more than the other.

Material selection and manufacturing methods can minimise TCR variations. The close proximity of resistor devices reduces differential temperature effects.

Operating Voltage

The dielectric strength of the materials used in the resistor network must withstand the operating voltages.

The insulation resistance between adjacent conductive traces or between the resistor elements and the substrate is crucial. Poor insulation can result in leakage currents or even short circuits.

In resistor networks with multiple resistors, voltage distribution across individual resistors must be uniform to prevent localised voltage stresses.

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. The voltage ratio for a given bit should be half that of the preceding bit.

Variations in resistor tolerance across the array, the TCR and drift in resistance across the array all impact on output accuracy.

Network resistors can be closely matched to eliminate tolerance issues. With resistor devices in close proximity on the same substrate, the impact of temperature variations is minimised.

Custom resistor networks are often required in low to medium volumes. This can mean they are not of interest to the major resistor manufacturers. However, custom thick film resistor network manufacturers focused on the application-specific marketplace can often help.