Be it pull-up resistors, pull-down resistors or just any other thing – many, if not most PCB designs come with a group of similar resistors. The knee-jerk reaction involves hitting the add a resistor button until the PCB is populated. Resistor networks provide an attractive alternative.
Resistor networks come in families. The most common one is simply a set of resistors which are connected in parallel to one another. A more exotic alternative, which is usually only offered by specialist vendors such as TTElectronics, connects one end of the packages together, thereby creating the bus architecture shown in the figure.
A bused resistor network can save time when designing pull-down resistor networks
We recently saw significant interest in a series of resistor networks created by Panasonic – a popular example would be the EXB-28V470JX. The Japanese vendor offers a wide variety of resistor values and usually provides three different configurations, all of which are shown in the future.
Panasonic sells resistor networks containing two, four and eight individual parts
Due to the common substrate, resistor networks do a good job at handling terminal problems. If one part heats up, heat dissipates to the remaining parts, thereby ensuring that drifting remains – at least to some extent – the same across all parts. Another obvious benefit is the smaller size – if you play SMD resistors very close to one another, space is wasted between the individual components.
When counting pennies for parts, the resistor network obviously is a bit more expensive – if only because the volume of resistor networks produced pales when compared to classic resistors. On the other hand, keep in mind that pick and place fees also cost money.
Sadly, nothing comes without disadvantages. The first problem with resistor networks is availability – while one can almost always find an SMD resistor somewhere, finding a specific network in the middle of a battle zone or an undeveloped country is difficult. The same applies to finding replacements if the vendor decides to discontinue the component.
When working with high-frequency or high accuracy designs, crosstalk can also become an issue-the common substrate, after all, can act weird as frequencies get high. Finally, keep in mind that a resistor network – by definition – places all of the resistors close together physically. This might not be a big problem for four or six layer boards, but can become critical when dealing with a space-constrained two layer board.