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I have designed a circuit for amplifying a PH probe signal and am unsure what type of ADC to use

I was reading on differential vs single ended ADCs. What is the best to use here? I know differential will give me some noise rejection. If I go this route, what do I use as a reference voltage? I do not understand how the differential signal gets converted to the digital values. Do I supply a reference voltage somewhere in the middle of the upper and lower bounds I expect, and it would use the +-Vdd or +-Vdd/2 as the max/min value??

With a differential ADC, can the difference in voltage between the two input pins be negative and positive? Do I not need to do a DC offset(as is done in the circuit above) to get me into positive voltage?

Can I use a single ended ADC here?

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It would be good if you characterised the signal from the pH probe. I've never used one so am not familiar with the type of signals they put out. I think they are a few mV and quite high impedance, but am not sure. I could look it up, but this is your project and therefore your job.

Let's assume you have a very high impedance signal to amplify, and the voltage range is +-100mV.

First, the 741 opamps you show are totally inappropriate. They have way too low input impedance. Please return them to whatever museum you found them in. There are plenty of good high impedance amps out there to chose from, but watch the input offset voltage.

As for the differential part, you have one side of the probe connected to ground, so you only have a single ended signal to start with. Ultimately you need a single ended signal if you're going to feed it into a microcontroller A/D. Some stand alone A/Ds can take differential signals in, but once you add your own amplification you will generally have a single ended signal anyway.

Your circuit only has a overall gain of 3 as shown. That would only result in 300mV peak if the input is really limited to 100mV. I expect the micro's A/D to want more than that for full resolution.

One characteristic of pH signals is that they are slow. This means you can apply significant low pass filtering, and can use a high resolution but slow A/D like a delta sigma type. These have enough resolution over the 100mV range that you don't need any amplification.

However, since pH probes aren't cheap and this is unlikely a high volume product, I'd make it easy on yourself. Get a decent instrumentation amplifier. These can be had with high input impedance, fixed and accurate gain, low input offset, and can generally level shift the result. A gain of 20 would give you a 4V output range, which should be about right for a 5V A/D. Level shift the output so that the 0 signal is at half the A/D range, and the rest is firmware.

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  • \$\begingroup\$ Am using LMC6041, see annotation on image-->my sim tool does not have very many parts. \$\endgroup\$ – user623879 Oct 19 '11 at 16:23
  • \$\begingroup\$ This may turn into volume product, and I am going to use standalone ADC...not overly worried about gain at this point, can fine tune that with some testing. With instrumentation amp, looks like a reasonable thing to do, though it is a bit more expensive, but do I need it? I only need res of around .01pH. This is hard to know without trying it \$\endgroup\$ – user623879 Oct 19 '11 at 16:36
  • \$\begingroup\$ @user: You still haven't characterized the probe signal. What pH range? What probe voltages does that correspond to? At what impedance? \$\endgroup\$ – Olin Lathrop Oct 19 '11 at 16:41
  • \$\begingroup\$ Wish i had the probe and access to a decent scope lol..but I want around 3-10ph range, or -240mV to 180mV range....ph probes by their nature require extremely high impedence..even nA currents can cause significant error. The interface device requires on the order of pico or even fempta amp input bias. \$\endgroup\$ – user623879 Oct 19 '11 at 18:08
  • \$\begingroup\$ Might end up using this: ti.com/product/ina116#samples \$\endgroup\$ – user623879 Oct 19 '11 at 18:14

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