Pull Up Resistor Noise

April 23, 2018

In digital electronics the engineers use the terms pull up or pull down resistor this is a simple resistor usually between 10k and 100k to define the input state when no signal source is connected this way overriding the noise signal common sense dictates that when you have potentially larger noise then a smaller resistor is needed but don. If the resistance value is too low a high current will flow through the pull up resistor heating the device and using up an unnecessary amount of power when the switch is closed.

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In other words it sources current from the power supply to pull the voltage up to a specified value typically the operating voltage so that it can be read as a logic high signal.

Pull up resistor noise. Leakage current as mentioned and capacitively injected external interferences noise set a practical limit to pull up resistor values. A pull up resistor is a resistor that works the chip normally high and change it to low when switched on pull up or pull down resistor mostly they are connected to the pins of the chip so that they can define the state of the pins. Think about ohms law a weak or high resistance pull updown resistor is good because it will not require much current from the driving circuitry to work against the resistor.

Usage of pull up down resistors to eliminate noise in digital circuit. Guide to use pull up and pull down resistor in your digital circuit. A pull down resistor works in the same way as the previous pull up resistor except this time the logic gates input is tied to ground logic level 0 low or it may go high by the operation of a mechanical switch.

Calculation of pull up and pull down resistor to use in your digital circuit. A strong pull updown resistor is good because noise currents from unintended coupling and emi will result in smaller noise voltages. Pull up resistor values are typically of the order of 10kohm resulting in an input current of around a 20 microamps or so whereas for a pull down resistor for the input voltage to be pulled to below 07v an input current of around 400 microamps is required meaning the pull down resistor needs to be around 1 kohm or less.

The ambiguity of the term noise shows up here. The basic function of a pull up resistor is to ensure that when there is no input a circuit takes on a default value. The first factor is power dissipation.

When where and how to use these resistors in your digital circuit to eliminate the floating state. The appropriate value for the pull up resistor is limited by two factors. The thermal noise increases with a larger resistance value while the current noise decreases.

The current noise of a resistor is described by the current noise index with a code number. Thermal noise will effectively not affect usual digital circuits also not with pull up resistors. This makes the comparison of different manufacturers possible.

This pull down resistor configuration is particularly useful for digital circuits like latches counters and flip flops that. Batteries will thus last longer. The way to measure resistor current noise is defined in norm iec 60195.