Re: High channel count (Ch = 40) DRL issues

Hi Jeremy

These sort of hacks are always difficult to implement and less than
optimal.  You will spend a fair bit of money on components, connectors,
etc. and actually this should be engineered properly.  The R 2500 or so on
having a prototype board (with tooling costs) made by Trax is not that
much, compared with all the other costs.  Obviously you would need to be
able to design your own PCBs.

Here are my comments:

1. I certainly would not use a single opamp to drive such a huge virtual
ground circuit.  All those 20 boards would parallel and be seen as a rather
low load impedance to that poor TLC277.

2. You would have to substantially redesign the power supply because of
your high overall current consumption. So in your case I would rather use a
+3.0V and -2.0V linear regulator, creating a dual rail power supply with a
REAL ground rather than a virtual ground.

3. Alternatively, keep IC201 on every analog board.  Use IC201A to create a
local (PCB) Vground, but remove resistors /modify the circuit around IC201B
to effectively disable IC201B.    In this case, only populate C213 on one
board but replace the 10nF C203 with a 100nF on every board.

4. I would suggest to only use a limited number of channels to partake in
the DRL feed.  Say, channel 1 to 4.  I think that often you will not use
all 40 channels...  any channel that takes part in the DRL feed and is not
used, is going to stuff you around.  Such channel you need to terminate and
you can't leave it floating.

5. In fact, in your case I don't think I would use the centre feed from the
INA to derive the DRL.  I would recommend having a separate CMS input which
you buffer and then only feed that to the DRL driver.  This is normally
what is done in systems with many channels.  Doing this, you never have to
worry about terminating any input.  If a unused input then floats and
saturates the INA that's OK.

6. EMG normally has wider bandwidth requirements, so you should change your
anti-aliasing filter.  But then you obviously need a ADC system that can
sample at higher rates such as 1024/2048SPS.

Are you going to use a National Instruments ADC card?  For many EMG
applications you need concurrent sampling.  Don't know if NI make cards
with many concurrently sampled channels.  Ni/Labview always gives me this
idea of pedestian speeds and this would normally exclude the need for
concurrent sampling.  But I don't really know their available hardware.
Just that it's expensive.  You would also need to isolate your equipment
somehow.

Remember this:  Your ADC will determine if you are going to stay with a
+3.0 / -2.0V power supply split, or if you are going to use the more
conventional +2.5 / -2.5V rails.  If your ADC can be feed a 4.095V
reference, you need to stay with the modeeg voltages.  If your ADC card
uses a 5V reference, you need to use +2.5 / - 2.5V.

Are you a buddy of Jeff Joseph?  Jeff is sitting a few offices away from me
:-)

Regards
Stefan

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