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Electronics => Electronics => Topic started by: Razor Concepts on April 15, 2009, 03:30:18 PM

Title: Fixing floating output
Post by: Razor Concepts on April 15, 2009, 03:30:18 PM

A sensor of mine has a floating output when not activated, and that wreaks havoc on the ADC readings. Would the proper way to fix the floating output be to put a large value resistor from the output to ground?

Could I use an extremely high value resistor, like a 10M? The higher the resistance the less it will affect the ADC reading, but such a high value may just make it think its still floating.

Title: Re: Fixing floating output
Post by: SmAsH on April 15, 2009, 03:39:22 PM

i would say a resistor between signal and gnd may fix this problem. generally 10K-100K is good but can you say "experiment"? experiment a bit and see which one works the best.  just out of curiosity what sensor is it?

Title: Re: Fixing floating output
Post by: Razor Concepts on April 15, 2009, 04:06:54 PM

Turns out high values of the resistor is bad too, so I'm just gonna use a normal pull-down resistor at 10k

Its a SoftPot membrane potentiometer... unfortunately the design has it so that when you dont press on it the output is not connected to anything  :-\

Title: Re: Fixing floating output
Post by: SmAsH on April 15, 2009, 04:57:08 PM

wait so is it like a pot that increases output as you press down?

Title: Re: Fixing floating output
Post by: Soeren on April 15, 2009, 06:13:10 PM

Hi,

Quote from: Razor Concepts on April 15, 2009, 04:06:54 PM

Turns out high values of the resistor is bad too, so I'm just gonna use a normal pull-down resistor at 10k

Its a SoftPot membrane potentiometer... unfortunately the design has it so that when you dont press on it the output is not connected to anything  :-\

If you mount pin 1 to the A/D-C, Pin 3 to ground/0V and pin 2 through a 10k resistor to Vdd, you will have the A/D-C tied at all times and the resistance can vary from 0V to Vdd/2.

If, on the other hand, you connect it as intended and add a 10k resistor from pin 2 to ground/0V, you just have to factor in that you have a resistance network as opposed to a single variable resistor and let the software compensate for the resulting nonlinear behaviour.

Btw. If you have a 12 bit A/D-C, you can add a 2k2 resistor between pin 1 and Vdd, to get 0V to ~4.096V.

Title: Re: Fixing floating output
Post by: protoadv on May 01, 2009, 09:44:52 PM

Have you tried a simple voltage follower?
See here:
http://www.ittc.ku.edu/~jstiles/412/handouts/The%20Voltage%20Follower.pdf (http://www.ittc.ku.edu/~jstiles/412/handouts/The%20Voltage%20Follower.pdf)

Hope this helps,
Proto Advantage
https://www.proto-advantage.com/store/ (https://www.proto-advantage.com/store/)