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 Plasma-enhanced chemical vapor deposition - Wikipedia).
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
Lamb:
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 Plasma-enhanced chemical vapor deposition - Wikipedia).
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
Internal resistance is infinite, or as near as makes no difference, 10^10 ohms or more at room temp - this is
usually the case for a CMOS chip.
Internal impedance is not infinite, since there are series resistances and sample capacitor to charge up. This
means that the source impedance should be 10k or less if switching between analog pins, though you can do
two reads in a row from the same pin to overcome this.
The ATmega datasheet has a section about the analog multiplexer and ADC that explains in more detail.
MarkT:
Internal resistance is infinite, or as near as makes no difference, 10^10 ohms or more at room temp - this is
usually the case for a CMOS chip.
The Atmega328p datasheet quotes 100 Mohms typical, so 10^8 ohms rather than 10^10.
MarkT:
This means that the source impedance should be 10k or less if switching between analog pins, though you can do
two reads in a row from the same pin to overcome this.
More effective is to add a delay between setting the multiplexer to the desired input and starting the conversion. 1us per 10K of source resistance is sufficient. The technique of doing 2 reads and discarding the first only works because of the overheads of the analogRead call (i.e. the time between the end of the first conversion and the start of the second). If the input voltage varies only slowly, then an alternative is to connect a 0.01uF or greater capacitor between the analog input pin and ground.
dc42, the 0.01uF capacitor is a great idea, under the conditions you state. It should completely swamp the capacitance of the Arduino's analog input.
If the signal varies more quickly than that will allow, you can add an Op Amp buffer, but then you need to worry about when the signal gets close to ground if you are using a single supply voltage.
Better to use a resistive divider alone. Good news - the sensor requires a 10k or greater impedance. If you use two 10k resistors, that is only 20k loading it down, but the Arduino sees 5k of source impedance. So you get a 2:1 margin for each.