PH Probe A/D Coverter ?

[shawn]

Ok guys this is what I did. When I think of a voltage divider I think of 2 resistors thus divide by 2 which does not get it done. So what I did was use a 3 resistor voltage divider to divide by 3 which puts me under 5v max output. The highest ph reading I need is a little over 10 for calibration purposes. Dividing the 0 - 10V ph reading by 3 gives me .00333 volt change for every hundredth in ph degree. So then to figure my Vref+ I used this (vref+ / 1024 = .00333)
Now my variable that is read from the ad port is my exact ph +/- a hundredth, which I can live with since measuring ph is pretty much a joke anyways.

My next question is, what is a good way to make a non drifting precision regulator. Accurate to the mV and will hold voltage in rm temp. I hate using pots and caps on LM317 regulators it always seems like they drift allot, is there a better way.

[shawn]

Another question for you guys involving my ph probe project. Right now my amplification circuit for the ph probe has pots for calibration. I wanted to add digital pots instead. I realize that the digital pots are low amperage but I am having a problem. I hooked up the pot and wrote a program to vary the wiper position, and it works fine.Its a 10k digital pot by the way and its a mcp 41010. But when I hook my 12v regulated voltage to it (PA0), they burn up, they burn up without the wiper attached to anything and without the other side of the resistor hooked up (PB0). I don't understand with just the 12 volts hooked up there should be no amperage. Is there a better digital pot that will handle higher voltages/amperages.

Shawn

[brid0030]

If you only want a single voltage divider you can adjust the values of the two resistors to get whatever level of voltage drop you want. Sorry if you knew this already, but that wasn't clear from your post. Just make the 'first' resistor twice as big as the 'second,' which is essentially what you do when you use three resistors

For a stable power source you could try a switching power supply. I have used a LM2672 by national semiconductor with good success--I think it is the simplest in terms of a low part count. If that does not give you enough stability, you could use two regulators: a switching supply followed by a linear regulator. Also, maybe a fixed linear regulator would give better stability than one that is adjusted using a pot.

Sorry I know nothing about digital pots.

[shawn]

brid0030

Thanks for the ideas. I hadn't thought about the first resistor being twice the size of the second resistor. That is a good idea, better idea. I've already scratched the voltage divider idea and have just lowered the gain of the probe amplifier. I'm going to look into the switching regulator, I have never played with one.

I have the amp working good now but the varying voltage on the vref pins is throwing me for a loop. I may have questions for you on the switching regulator. The vrefs need to hold within preferably 1 or 2 mV, which is maybe an unobtainable goal. For my temp probe I am going to need 1.024 v on Vref+ and 0V on Vref-. This gives me 1mV resolution for my temp probe, a LM34.
For my ph probe I need Vref+ to be 3.41V and Vref- to be 0V.
For my orp probe I need Vref+ to be 3.354V and Vref- to be 2.333V This again gives me a resolution of 1mV. The reason Vref- for the orp probe has to be 2.333V is because the ph probe and the orp probe will both be in the water at the same time and they are both referenced at 2.333v, which is a virtual ground because both probes can read + or - . Both probes have to be at the same potential. This Vref problem I am having is a stupid problem but I am pretty sure it is obtainable even for a novice like me.

Thanks Shawn

[brid0030]

You appear to be groping your way through. Sounds familiar. I'm no expert on power supplies, so I don't know how much stability is improved by a switching supply. Fortunately, they are pretty easy to set up (though not as easy as a linear regulator). On the national semiconductor webpage, you can find a web-based and a downloadable tool for designing a power supply. Both with present you with various options and output a parts list and a circuit diagram for you. By the way, most of the components are available as free samples.

Can you get over the wobble in the Vrefs by sampling over a longer period, or by averaging lots of ADC readings?

[spec]

im busy to connecting a ph sensor to a controller
but searshing for a solution for a convertor to 0-10V

is there a way to help me?
regards