Is pull-low resistor needed at IC input?

[eepty]

I found some circuit has pull down resistor between IC's input pin and the ground, but some other circuit tie the pin directly to ground.

I would like to ask is the pull down resistor needed? Any pros and cons?
(Assume cost is not an issue)

Thanks!

 

[KlausST]

Hi,

I assume you ask about unused inputs.

In case there is a chance i need the pin later, maybe for some debugging, then I add a resistor. Else I connect it directely to GND.
(only with split GND planes, high power applications, or very noisy environment...maybe)
Technically i don´t think it has much benefit.

Klaus

 

[andre_luis]

Circuits having high impedance inputs are prone to suffer with antenna effects along the routed tracks and the pull-down resistor ensure that some noise will be drained to the ground.

 

[eepty]

Hi,

I assume you ask about unused inputs.

In case there is a chance i need the pin later, maybe for some debugging, then I add a resistor. Else I connect it directely to GND.
(only with split GND planes, high power applications, or very noisy environment...maybe)
Technically i don´t think it has much benefit.

Klaus

If it is a high power and very noisy application (an 9000W inverter), how it benefit to add the pull low resistor?

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Circuits having high impedance inputs are prone to suffer with antenna effects along the routed tracks and the pull-down resistor ensure that some noise will be drained to the ground.

Hi, could you advise some reference? Some of my friends said the pull-down resistor will make the pin to be susceptible to noise, because it is not at the same potential with the ground.

 

[KlausST]

Hi,

If it is a high power and very noisy application (an 9000W inverter), how it benefit to add the pull low resistor?

With high power, high current applications there is noise on the ground. And especially when you have split GND planes, there you will see ground bounce. If now the bounce voltage is so high (maybe onle short switching spikes) that internal protection diodes become conductive, then a resistor helps to limit the current.

Klaus

 

[andre_luis]

Some of my friends said the pull-down resistor will make the pin to be susceptible to noise, because it is not at the same potential with the ground.

In a circuit having for instance the gate of a CMOS transistor at input ( which impedance lies much above the range of houndreds megohms ), a pull-down resistor ensures that the smallest static electricity would be carried down to ground.

 

[eepty]

In a circuit having for instance the gate of a CMOS transistor at input ( which impedance lies much above the range of houndreds megohms ), a pull-down resistor ensures that the smallest static electricity would be carried down to ground.

If so, why not just tie the pin to ground directly? This will even ensure that there is no charge on the pin.

 

[andre_luis]

If so, why not just tie the pin to ground directly? This will even ensure that there is no charge on the pin.

This way you will not be able to use that pin as an input anymore. By using a pull-down resistor, you ensure that its voltage will not freely fluctuate at any logic level, but will stay at a predefined value.