From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-02-27 23:09:25
Hi,
----- Original Message -----
From: Andreas Robinson <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Wednesday, February 27, 2002 3:29 PM
Subject: [buildcheapeeg] My project - status report
...
> Test setup:
>
> The amplifier PCB (1mm thickness FR-4, 35u copper) was
> just laid out on the workbench without any shielding
> whatsoever. Power (+/- 5 volts) was supplied from a
> benchtop linear power supply using 60 cm (2 ft)
> unshielded cables.
>
> An oscilloscope was connected to the output of the
> amplifier board using 60 cm coaxial cable. About 10 cm
> from the end the coaxial cable is connected to two
> unshielded, but twisted, wires, in turn connected to
> the PCB.
Sounds good so far.
> 1. Testing the filter - without an instrumentation
> amplifier mounted.
>
> A function generator was connected to the filter stage
> using the same kind of cable used for the
> oscilloscope.
>
> Results: Ok, the filter works - no surprise there.
In an Tiny EEG amplifier schematic (dated 18.01.2002)
seems to be wrong value for R119 (0R).
I assume you have corrected this ?
(BTW. where is the most recent version of the schematic ?
I could not find one in the file area)
> Turning down the signal amplitude on the generator to
> zero (or at least very low) I saw repeated bursts of
> HF-noise, 6mV peak to peak with quiet areas in
> between. It probably comes from the computer nearby.
Simply turn the computer off to verify this assumption ;-)
> 2. Testing the whole amplifier - floating inputs
IMO not a very good test. The bias currents will
saturate the LT1167 inputs.
> The inamp was mounted and the gain was set to 50 with
> two 470 ohm resistors. The total gain was then around
> 1200.
Seems not to be very critical.
> The inputs were left to float, without any wires
> connected to the PCB.
> The 100pF HF-chokes were floating as the input stage
> shield was not connected to system ground.
>
> Results: Fairly random 50mV(p-p) LF-noise at the
> output (7.5 bits resolution, wee! :-)
>
> Connecting the shield plane to true ground in a single
> point on a direct line between the 100pF caps and the
> amplifier ground input halves this noise (8.3 bits).
>
> By the way: When I touch the amplifier inputs once,
> the noise is replaced by a high level (200mV) 50Hz hum
> that remains even when I remove my hand. The 50Hz
> signal stays until I power-cycle the amplifier.
IMO this test says nothing.
Try connecting the inputs via 50k resistors to AGND
or the DRL output to simulate electrode resistances
and to provide a bias path.
> Attaching 3cm (around 1 inch) leads to the amplifier
> inputs causes the hum to appear immediately at
> power-up.
>
> 3. Testing the whole amplifier - shorted inputs
Better test conditions...
> The inamp-gain was increased to around 100, so the
> total gain is now about 2500.
> Simply shorting the inputs to ground leaves a very low
> level (1mV) 50Hz signal and some HF-noise of lower
> amplitude (maybe 250-500uV) on the output. This means
> the amplifier has an internal dynamic range of at
> least 12.4 bits in an unshielded environment and with
> standard lab-power.
Calculated for what amplifier input range and what ADC input
range ?
> That's how it stands at the moment. Testing with
> electodes will have to wait until I can run the whole
> thing on batteries and see the results on my computer
> instead of an oscilloscope.
>
> The ADC+MCU is assembled, but has only been
> "smoke-tested" so far. (That is, it didn't catch
> fire.)
> The board is not flat enough to fit in the scanner or
> I would have attached a picture.
> The IR-receiver is about 50% done, because I made a
> big mistake, using the narrow SO footprint for the
> MAX232 when I should have used the wide, so I'm saving
> that particular mess for later.
>
> Power is not done yet either. A group member named
> Scott Elofson was kind enough (thanks again Scott!) to
> send me two +/-12V (500V isolation) DC/DC converters
> to play with. That sort of got in the way of the
> battery approach. I have built a working board (with
> patches. :-p) with one of the converters, but I'm not
> sure I've managed to get the noise levels down
> sufficiently. 2mV HF noise is very close to being
> unacceptable.
Where did you measure the noise ? At the opamp power pins ?
Did you use a differential probe or a single (referenced to
earth) probe ?
> The RLC-filters are not nearly as
> efficient as calculated for some reason...
Provide schematics / layouts. Then I can say more.
Regards,
Joerg
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:39 BST