Does analog time division demultiplexing need any additional sample and hold?

Question

Quick general summary: In analog time division multiplexing, after combining several continuous signals into a single line, do you generally need to use some form of sample and hold to reconstruct the continuous signals on the demux output side? Or do mux IC's do this automatically?

My specific application: I'm collecting 256 voltage signals from tiny wires embedded in brain tissue, eventually collecting them on a DAQ card and doing online analysis. The multiplexer (perhaps ADG1606 on both mux and demux side) is intended to reduce the wire-count in the long, expensive cable bundle that connects the on-head signal buffers to the rest of the amplification and digitizing hardware (please see figure - I hope the size is appropriate). From the answers I've gotten so far, I drew what seems to be the necessary general idea, and I'm wondering about specifications and unforseen limitations.

For the sample-and-hold, I'm considering this setup, except that the droop rate (2mV/ms) and sample time (3ms) seem bad for my application. Maxim seems to offer inexpensive SH packages with better specs (DS1843 DS), would you recommend using those instead of making SH stages from opamps and switches as the tutorial outlines?

Am I right in imagining that independent SH circuits synced with the mux timing will reconstruct the original independent pre-multiplexed signal (at least the lower-frequency parts that I care about - 0.1Hz to 9kHz)? Or does the SH switching introduce periods of corrupt output data?

May I skip my late 5000x amplification step (256 channels) by 5000x amplifying the 8 mux lines just before demux? This would save me some space and money.

And, do I need to lowpass filter the brain signal BEFORE it gets to the mux? I have not done this in my current setup (which doesn't have any mux), but with mux I think I may need to worry about aliasing?

You may be able to tell that I'm new to this, so clues that may seem obvious to you are still very much appreciated!

Recording setup diagram here and below

Answer 1

If you are going to continue to use 256 amplifiers, ADCs, etc. you will need a sample and hold that is triggered each time you activate the mux channel. One thing you'll have to pay attention to is the droop rate of the SH.

Alternatively you could now get away with a single amplifier, ADC, etc. for each group of 4 or 16 channels. You would know which channel was being sampled because you are driving the mux to select the proper channel. This would greatly reduce the system complexity if you are able to do it.

Note for any of these configurations you will now have to send several signals to your mux to select the proper channel. This means you'll need some device to handle the sequencing, and this device could also be used to handle the triggering of the SH amplifier, or the per block ADC.

Answer 2

A few comments:

• You really want to do your amplification before going through the MUXes. Each mux will introduce some series resistance, as well as causing small spikes when it switches (see the datasheet, it's described as "Charge Injection"). Furthermore, the mux has some distortion, as well as non-linearities.
  Basically, you want as big a signal as possible, to reduce the contribution of the multiplexers.

• Why are you demuxing at all? Most NI DAQ systems have their sample rate described as "Aggregate", which means that internally, they have a single channel ADC, and a bunch of muxes. As such, the DAQ switches the ADC between channels very rapidly, to sample multiple channels.
  If you have a data acquisition system where the sample rate decreases as you use more channels, it's probably internally multiplexed already.
  The only advantage I can see to demuxing before entering the DAQ is that it lets you put the filters there, rather then at the preamp.
  On the other hand, a simple 3 or 6 dB per octave filter is very compact, so it should be pretty simple to put it at the preamp end.

If you have long wires (and you do), you should really consider using differential signaling. With the signal levels you are working with, you will have EM pickup issues is you use single ended signaling, particularly if you do not amplify the signals before sending them through the cable.

Also, shielding the 3m cable is a must.