[dorkbotpdx-blabber] Dealing with floating analog inputs, resistor value

[Jared Boone]

Hi Alex,

For your average CMOS IC, you only need to put a few microamps into  
the pin to nail it down. Look in the data sheet for an idea what the  
required input current is for a high or low voltage level.

Take your power supply voltage (say 5V) and divide by maybe 10uA  
(Ohm's law) to figure out a reasonable resistor value:

	5V / 0.00001A = 500KOhm

I think people tend to pull up, instead of down, because the distance  
from the supply voltage to the threshold voltage is usually larger  
than between the threshold voltage and ground.

In practice, once the input is pulled up to the supply voltage, it  
dissipates virtually no current, as the voltage difference between the  
supply and pin voltages is miniscule (thus the current across the  
resistor is also miniscule). Probably picoamps.

Here's a good summary:

	http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html

	- Jared

On Jul 30, 2008, at 3:16 PM, Alex Norman wrote:

> Hi, I have some analog inputs [an i2c analog chip, max127 or  
> max128..] that will
> be connected to a 1/4" [or maybe 1/8"] phono jack, allowing for  
> various sensors
> to be plugged in.
>
> Obviously I don't want these inputs to be floating when nothing is  
> plugged in,
> so I figure I should tie the analog inputs through a resistor to  
> ground.
>
> My question is, how should I determine the value of this resistor?   
> I figure a
> high value resistor will reduce the power consumption... what is the  
> trade off
> there?
>
> Thanks,
> Alex
>
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