PH Probe A/D Coverter ?

[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