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Biopotentials, Instrumentation Amps + Multiplexer

Question asked by AndreasG on Apr 11, 2013
Latest reply on Apr 12, 2013 by seanbrown

Dear Forum,

I am looking for some guidance with setting up a preamplifier for biopotentials (10µV to 100 µV signal range, BW 1Hz to 3 kHz). The electrodes have a  quite high impedance (@1kHz) of 1 MOhm. The gain should be between 500 - 1000.

The signal will be digitized  at 10 kHz with an NI-PCI6040e DAQ card, with 12-bit resolution with the tiniest input range around  +/-100 mV.

For this purpose I've used so far an AD8221 for each channel with good results. Now I'd like to expand the number of channels (~8 channels) but minimize the space (something in a LSCFP format?). So I thought about multiplexing the electrode inputs. But honestly I have no clue where to start and how to avoid pitfalls.

 

Only the +IN will be multiplexed, the -IN is fixed to one electrode.

 

1) I don't know if there's some kind of switching noise due to the multiplexer and if I should amplify the signals before I route them to the multiplexer to increase the SNR signal/switching noise? And as this noise would only occur on one channel, should I rout +IN and -IN through the multiplexer?

 

2) as DEMUX I use ideally the same IC as the MUX, right? and control both IC's with the signal out of one signal source.

 

3) I would use at least 30 kHz as multiplexing frequency, right?

 

4) I guess it is quite important to reduce any capacitance at the input path

 

5) does it make sense to switch inbetween every channel-switch always to -IN to aid settling?

 

6) is it a straight-forward approach or should I implement the whole story differently?

 

 

Thank you in advance,

Andreas

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