summary
Sending analog audio signals a few meters over shielded coax cable is a solved problem.
If your signals are limited to 3200Hz and you need 8 or 10 bits of precision per sample, then I would be pretty comfortable using standard audio coax cable to send the raw analog signals.
That might be the lowest-cost, lowest-battery-power way to handle things.
If you require DC-accurate readings at 3200Hz and 20 or 24 bits of precision per sample, shipping analog signals over even 2 meters of cable is basically impossible at any price.
If you need that precision, you are forced to digitize the signals right at the source, and ship them over the cables in some digital format.
details
Transmitting in digital format generally requires one to spend a little more money on electronics at each sensor, but it allows you to save a little money on lower-cost UTP cables and low-cost connectors.
In a few cases, transmitting in a digital format lets you use fewer cables -- a single daisy-chain through each sensor ending at the host, where each sensor forwards data from the "upstream" sensors "downstream" towards the host computer, as well as sending its own data "downstream" towards the host computer on the same cable. With an analog system, you are pretty much forced to run an independent wire to each sensor -- analog multiplexing techniques end up costing more than digital multiplexing techniques.
As the bandwidth goes up, or the desired precision of the signals goes up, or both, analog cables need more and more shielding (i.e., get more expensive) to block outside interference and cross-interference.
Eventually you reach a point where it's basically impossible to put enough shielding on a cable to get the desired bandwidth and precision.
Suggestion
Post a new question something like "I have a bunch of (insert name here) digital sensors that I want to distribute over a large area of a few meters, but I don't want to run dozens of digital control wires per sensor back to my host MCU. What's a reasonably low-cost, low-energy digital circuit I can put to reduce the number of wires I need to run to each sensor? I might be willing to run a full 4-pair CAT5 cable to each sensor to carry power + data, but no more!
Ideally much less -- is it possible to share one 4-pair CAT5 cable among 2 or 3 sensors to carry power + data?"
If you are willing to spend a few extra bucks on digital chips in order to avoid the hassle of programming a MCU at each remote location, please specify "without a MCU" (like How to decode morse code with digital logic ).
It's possible the resulting circuit may give you the full precision available from your (insert name here) digital sensors, but have a net cost less than an analog-signal system, when you balance the extra digital electronics and the low-cost unshielded cables and connectors vs. the lower analog electronics cost and the higher shielded cables and connectors.
I don't think that multiplexing raw pH inputs using analog switches is such a cool idea. They are high impedance, and the probes can be destroyed if the muxes latch up due to ESD etc. It won't cost a lot to have a buffer amplifier on each of the 4 inputs, and then hook up a multi-channel A/D converter to their outputs. No need for a mux, and multichannel A/Ds are a dime a dozen.
By multiplexing the input to an amplifier you're also requiring the amplifier to have much higher bandwidth than otherwise necessary, and it might be impossible to design such a circuit while still having acceptable noise.
In your case, you want to take 400 samples/s in total - you're sampling at 400Hz. Your mux would be switching a Ph probe each 2.5ms to the amplifier. Assuming an A/D with 100us acquisition/conversion, the amplifier has to settle to A/Ds resolution within 2.4ms. Let's say we want to settle to 1/2LSB on a 14 bit ADC. It requires at least 10 time constants of the circuit used:
$$n=-\log{1\over{2^{\mathrm{bits}}\cdot \mathrm{LSBs}}}=-\log{1\over{2^{14}\cdot 2}}=10.4$$
This means that the time constant of the amplifier needs to be:
$$\tau={2.4\mathrm{ms}\over10.4}=0.23\mathrm{ms}$$
The amplifier needs to be a 1st order low pass with the cut off frequency greater or equal to:
$$f={1\over\tau}=4.3\mathrm{kHz}$$
This bandwidth is 100x what's needed by the pH probe alone. Your multiplexed amplifier will have ~100x the noise amplitude compared with one that had a 16Hz low-pass response and were be applied to each pH probe directly.
And all this assumes that you are using an ADC that's much faster than the application calls for (10kHz sampling rate for 100us acquisition/conversion), although admittedly that's not a problem with a low resolution ADC.
In closing: Compared to placing the multiplexer after the preamplifiers, the multiplexed amplifier design requies an ADC that can sample 25x faster than 400Hz, and an amplifier that produces 100x the noise. If you can live with those drawbacks, you're OK.
Best Answer
Seems like a sound approach to me as long as you are working with low voltages and sea-level/room-temp environment. You should make sure you understand the impedance of the SATA cable and that you are matching the impedance of the traces on your board and that you have a matched load. This is especially important if your signal includes high frequencies.
Also, you may want to make sure that nobody confuses your connector for a normal SATA connector, especially if there is potential to damage your system by doing so. I would consider adding some sort of additional keying, if possible, or at least add a warning label.