Multiplexing analog signal

Question

I want to read 4 analog signals (coming from 4 pH sensor, generally said 16HZ freq.) With one buffer op-amp circuit going into a single ADC channel. Is it possible to achieve this using analog MUX ICs? Im asking because I'm not sure if the MUX will distort the analog signals. If yes, what other IC's can you recommend that is able to take 100 samples per second for each channel?

Answer 1

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.