From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-02-04 21:10:47
Dear Joerg and Andreas:
> 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.
Joerg, I fully agree with you. I made sure that Andreas provided a way on his PC board to defeat this option. I would not want to use it. This is basically an auto zero integrator, not an AC coupled stage. I would love tosee a step response simulation on that.
With a coupling capacitor and resistor to ground you can do tricks ( like diodes or momentary short to ground to charge the capacitor ) to improve theoverload recovery time. Not so with the integrator.
I would also caution about running these stages at such a low power supply voltage. It invites clipping problems. I ran my input stage at +/- 5 volts, but found significant improvement when I went to +/- 12 volts. Theoretically it should not matter.
This auto zero integrator would provide no benefit if the power supply voltage were higher.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
----- Original Message -----
From: Joerg Hansmann
To: buildcheapeeg_at_yahoogroups.com
Sent: Monday, February 04, 2002 11:34 AM
Subject: Re: [buildcheapeeg] Re: Brainmaster input stage
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
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:38 BST