PH Probe A/D Coverter ?

[shawn]

Another question to help me solve this problem. When I first started playing with the VREF on the pics I thought you could raise vRef+ above vdd, well looking at the data sheets I learned I was wrong. My next question is can I raise Vref- above ground, say 1.33v or does vref have to be ground. The reason I ask, is because if I divide the output of ph probe amplifier by 3 then I should be able to get the range I want out of it in a 2V swing. If I can set vref+ to 2.354V and Vref- to 1.036V I will get a 2mV/Bit of resolution which I think will work for what I want to do.
It would still be nice to find an AD chip so I do not have to use voltage dividers.

Shawn

[skimask]

My next question is can I raise Vref- above ground, say 1.33v or does vref have to be ground.

Yep, you can put Vref- to anything you want (>Vdd not included )
If your crafty enough, you can even use a couple of the PICs pins to make an R-2R ladder so a guy can even vary Vref+ and Vref- to suit the conditions. (i.e. read 0 on the A/D, lower Vref-, read 1023 on the A/D, raise Vref+, and so on...)

[brid0030]

Not a real answer to your hardware questions, but could you rig up a simple manual switch that lets you select from among 3 or 4 voltage-divider arrangements? Whenever I've used a ph meter I have always known whether I had a strong acid, something more or less neutral, or a strong base before getting an exact reading. If you have a clue as to what your are testing beforehand and can anticipate a reasonable range of output, you might be able to tailor the voltage divider to the output and get a lot more out of the 10-bit ADC.

[skimask]

but could you rig up a simple manual switch that lets you select from among 3 or 4 voltage-divider arrangements?

That's what I was saying with using the extra pins.
If you take the extra pins, bring them out, connect a 1K (for example) in between each pin, put +5v and the top, ground at the bottom of the string, and change the pins from input to output (or back), and high to low (or whatever), you can dynamically change the Vref (+ and -) points without having to slide a switch...let the software do the work. Start wide and dial the reading down to a tight reading.
For that matter, you could even use a couple of digital pot's as a variable voltage divider under software control.

[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.