EMC Question of the Week: January 25, 2021
A 1-kΩ resistor in series with a CMOS input pin on a microcontroller can help to improve
- conducted emissions
- radiated emissions
- ESD immunity
- all of the above
Answer
The best answer is “d.” Internal noise generated in a microcontroller (or any digital integrated circuit) can be conducted from the IC to the board on any of the pins (including input pins). A 1-kΩ resistor in series with an I/O pin helps to contain high-frequency noise, which can lead to reduced conducted and/or radiated emissions. The same resistor can help to improve ESD immunity by limiting the bandwidth of the input. For example, if the input capacitance is 5 pF, a 1-kΩ resistor increases the time constant to 5 nanoseconds. This effectively prevents most field-coupled ESD events from inadvertently triggering the input.
Of course, a 1-kΩ resistor can't be placed in series with inputs designed to operate at data rates in excess of about 30 Mbps without impacting signal integrity. Nevertheless, low-speed I/O pins are often the primary path by which unwanted noise is coupled to or from an IC. Series resistance is a powerful and inexpensive means of controlling this noise.
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