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Author Topic: how to manage an analog input from a 0to10V source  (Read 1230 times)
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Hi
I am working on a lab project at the I.N.R.I.M. where I am configuring an arduino to control a PECVD proces.
(off topic if you are intrested
http://en.wikipedia.org/wiki/Plasma-enhanced_chemical_vapor_deposition).
Among other things I need to monitor with a baratron the pressure in a vacuum chamber and my baratron (mks 626a type) gives me an output from 0 to 10V.
What i have in mind is to use 2 resistors with the same impedence and wire arduindo between the two resistors for the analog input and wire the ground of arduino to the signal ground of the baratron.
The problem is that I cannot figure out what is the internal resistance of arduino, if my 2 resistors are not small enought in comparison to the internal resistance of arduino my voltage will drop down.
In the specifics of the baratron the output signal has no current intencity specified so i am guessing it is aproximaly 0

* scopemtbumeng0400.pdf (2212.66 KB - downloaded 25 times.)
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sorry the atachment was wrong

* Baratron 626.pdf (551.08 KB - downloaded 92 times.)
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Hi, do you mean 626B type?  There is no description I could find of a 626A in that attachment.

Geoff
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Yes i ment the 626b type
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Hi,

I'm not sure you need to know the internal resistance of the analog pin on the Arduino itself as it's not a continuous connection to the input.  What's there is a small capacitor that is connected and charged for a short time, then the pin is disconnected from your source and the capacitor is sampled to arrive at the analog reading.  In the ATmega328 datasheet:
Quote from: ATmega328 datasheet, section 24.6.1 Analog Input Circuitry
The ADC is optimized for analog signals with an output impedance of approximately 10 k Ohms or less. If such a source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely.
Your instrument specifies a load of 10k Ohms or more, so if you used a resistor divider with two 10k or greater resistors it would be satisfied.  To reduce the input impedance to something the Arduino analog input will prefer, you could simply include a 0.1uF capacitor from that analog input pin to GND so when the sample is taken, it's from that already stored voltage in the cap.

There's a discussion of something similar (though a more extreme resistor divider is discussed in this thread where other options are proposed also.  From that thread you'll see it's quiite possible if your sample frequency is low enough you won't even need that cap.

Cheers !
Geoff
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