How to calculate the pull down or pull up resistor value

Explain how to determine the pull down and pull up resistor value

Usually, CMOS IC’s require a pull-up or pull-down resistor as their outputs cannot source current from the internal supply as can TTL. For that reason, you need to understand the load of what they are supplying, use the IC specification sheets to ascertain. Check fan out for all loads and use buffer if required. The nominal current will often be around 30mA. Nominal CMOS voltage will be 12v as opposed to the 5 V used on TTL and you can then use Ohms law to calculate the resistance required.

There are several reasons why you would use “pull-up” and “pull-down” resistors in Digital logic and Analog Circuit designs. Both applications will be reviewed here.

In DIGITAL communication application, it is better to pass a small amount of CURRENT along the long piece of wire than just to toggling the voltages between 0V and 5V signal from one end (example: RS232 Serial Communication protocol - max. distance of 50 feet at 20 kHz).

Therefore in order to pass the current, you would need either a pull-up or pull-down resistor (current source) at the receiver end of the wire while a logic signal (current sink), “open-collector” type driver is used at the sender end of the wire. However, the current can be passed either to the source or to sink (pull-up or pull-down) depending on the type of switching between 5V and GND (0V) signals.

The resistor value depends on the amount of current needed to source or sink to overcome the CAPACITANCE within the closed-loop circuit. Any capacitance, either inheritant or deliberately placed, will delay the switch signal which can be calculated (RC time-constant).

In ANALOG applications, many 2-wire sensors can be resistive type (example: temperature sensors) which also requires either pull-up or pull-down resistor to create a voltage divider network in order to sense the voltage drop across the sensor element. In this type of application, the current is also passed along the wire (from the sensor to A/D converter) to minimize noise, similar to the digital communication circuit as mentioned (above). Pay extra attention to the RC filter (the time-constant) when designing analog circuit (LSD of A/D converter) as well.