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

Hi Jeremy

1.  Every INA and every TLC277 consumes about 1mA.  So in total you have
about 4mA per channel. 4mA x 40 channels = 160mA.  That TMV0505 can give
200mA.  Mmm, a bit tight but I guess you can do that... but you cannot
isolate your analog channels, so there is no use in isolating the power
supply at that point.  I still maintain that a true dual rail supply would
be better in this case.  Be it for a much lower ground impedance than a
virtual Vgnd circuit can supply.

2.  I think I follow you - sounds workable, but not ideal.
Engineers are "picture" people :-)  A picture (schematic or block diagram)
would make it easier to follow.  But I know that this forum doesn't easily
lend itself to such distribution.

3.  I never had the privilege to rip apart a many-channel commercial
amplifier... But I am sure they wouldn't do the DRL that way.  FIRSTLY,
they would NOT use differential inputs with INA amplifiers. It would likely
be a monopolar design from the outset, only using opamps.  If you work with
16/32/64/128/... etc. channels, things are just done differently due to
cost and practical considerations.  I think these systems use a separate
CMS input electrode and use that solely to peg the DRL drive signal.

4.  Yes, you should have that covered.  You can simulate the circuit with
whatever new component values you have chosen.

5.  I just checked the datasheet on that DAQ card.  They say it can handle
+/-0.2, 1, 5 and 10 volt bipolar input signals.
If I understand you correctly, you want to connect the ADC's ground to VGND
(+2V).  That would produce asymmetrical clipping of the input signal.  It
would be better to then make VGND = 2.5V.  The output of your analog
section would have a Vout max = 5Vpp.  But your DAQ has a FSD output with
10Vpp input (assuming you use the +/-5V sensitivity).  So you would loose
one bit out of your 16.  OK, 15 bits is ample.

This analog to DAQ interfacing is the most difficult for
beginners/youngsters/students to understand.  There are many design
decisions that need to be made.  It will have noise, headroom, dynamic
range, signal offset and symetrical signal performance consequences.  One
would need to look what is going on on that USB-6225 card and take it's
input circuitry design into consideration.

You didn't say what isolation you are going to implement.  If it is lab
equipment you could do without, if it is strictly understood to be run from
a battery powered laptop WITHOUT any LAN or mains powered printer
connectivity.  But I would not be in favour of this.  The temptation to
take shortcuts and/or forget is just too big.

You could consider isolating that USB line with the ADuM3160/4160.  And
additionally, also ensure that your DC power is save, from wherever you
might source it.

Have fun.

Regards
Stefan

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