From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-02-04 16:34:57
Hi Andreas,
----- Original Message -----
From: Andreas Robinson <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Friday, February 01, 2002 5:16 PM
Subject: [buildcheapeeg] Re: Brainmaster input stage
...
> > > By the way, why don't you use an integrator
> plugged
> > > into the REF-input?
> >
> > I have seen this sort of hp in the datasheets and bm
> > schematic and do not understand the point.
> > IMO it is effectively a hp-filter. However I could
> > also get the same characteristic with a simple RC
> > hp. So why waste an additional op-amp ?
>
> Ok, I've attached an image that might change your
> mind. It shows two instrumentation amplifiers, running
> at +/- 2.5V. Both have G=100 (approximately), but one
> is DC coupled and the other AC coupled using an
> integrator. I'm feeding both a 1mV signal with a 25mV
> offset.
>
> The one with DC coupling is saturated, while the other
> one is still working. As you pointed out, this is not
> a true AC-coupling effect, as both amplifiers are
> saturated at 30mV. Anyway, the DC-coupled version
> begins to distort the signal at around 15mV offset, so
> you get an extra +/-10mV (66% increase) offset range
> without lowering the gain or using hp-filters (with
> noisy resistors) at the inputs.
Interesting. It must be a problem with the output swing
of the output stage of the LT1167.
To verify this I have tapped the internal pre amp nodes
(out1 and out2 ) of the LT1167 simulation model and plotted the
output voltages at different common mode and reference
voltages vs. the input difference voltage (x-axis).
(see attached picture)
It can be clearly seen that the internal pre amps have
a greater output swing than the output stage.
However in a single supply 5V system with a ADC input range
of 0..4V a second single rail stage (like TLC272) with adequate
gain (e.g. G=10) has to be used anyway to get down to the
negative supply rail.
In this configuration the reference integrator would make no sense.
So I continue to claim, that the reference integrator hp-filter
has very little advantages compared with a simple R-C high pass.
...
> Well, you can have a HP48 too! Sort of. It is out of
> production, but there are emulators... (that are
> faster than the original)
>
> A beginner's page:
> http://www.area48.com/index.html
>
> Latest emulator version:
> http://privat.swol.de/ChristophGiesselink/emu48.htm
Great ! I have played around a little bit with it and
find that it will require some time to learn using it...
Regards,
Joerg
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:38 BST