From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-03-02 01:09:01
Hi Andreas,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Thursday, February 28, 2002 4:55 PM
Subject: [buildcheapeeg] Re: My project - status report
...
>... the latest schematic is uploaded now.
I have just downloaded them.
...
> As you can see some things have changed, and some things need to be
> changed. I hope I have not added any bugs... Ok I know I may have
> added one bug, that may already have reared its ugly head, but I can
> patch it if necessary.
>
...
> > > 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.
>
> Do you mean touching the amplifier?
No. Doing tests with inputs floating.
...
...
> > Did you use a differential probe or a single (referenced to
> > earth) probe ?
>
> Single-ended ... :-p (Anyone got a FET-probe lying around?)
If you have a 2-channel scope you have the option to add both channels
while one is inverted. If you use the same gain for both channels
you effectively get the difference between both probes.
This setup eliminates much of the ground noise without
need to have expensive FET probes ...
> > > The RLC-filters are not nearly as
> > > efficient as calculated for some reason...
Did you include series inductances of some 10nH for each C of your simulation ?
> >
> > Provide schematics / layouts. Then I can say more.
>
> Ok, it is uploaded to the files section. The DC/DC converter is not
> specified on the schematic, but I'm using this one:
> http://www.dcdc.com/pdf/ha&hp.pdf (model HP22-060-18)
>
> Some notes:
>
> I tried patching the circuit by paralleling the 2200uF cap with a 1uF
> film cap - no difference at 20MHz (where the disturbance is).
For 20MHz you need 10nF ceramics (with very short wires) very near
to the DCDC outputs.
Treat your bypassing Cs in your layout as if they had 4 poles
(2 input wires and 2 output wires)
The layout traces of a bypass C should look like a "X" and not
like a "H" to minimize trace inductance in series with the C.
(each cm of trace has about 10nH)
> Most of the noise comes from the switching spike generated in the
> power supply, which is a Mascot 9921 medical power supply:
> http://www.mascot.no/pdf/9921.pdf
>
> Just measuring its unloaded output shows a single spike occurring at
> the switching frequency (40kHz).
>
> Connecting it to my circuit, produces terrible ringing at around
> 20MHz. I could successfully simulate the ringing in spice - the cable
> works as a transmission line (1.8m unshielded cable) with a 20MHz
> resonance peak.
>
> I tried terminating the cable by using an impedance (22 ohms + 100nF
> in series) across the terminals but it didn't help.
Try to reduce the problem at its source.
> Coiling the cable and putting my hands around it reduces the ringing
> visibly (about 50%), so I'm guessing a large ferrite bead like the
> ones used in VGA-cables could help here? Unfortunately, my local
> supplier (ELFA) is permanently out of stock, of the models I need
> (though they catalogue them), or I would have tried it already.
BTW: For dual power supplies it is always a good idea to put a reversed diode
(capable of handling the regulator output shortcut current)
from each regulator output to GND. Otherwise in case of a pcb failure with
shortcut between +5V and -5V one regulator will win and pull a part of the
attached circuit to reverse polarity - and destroy it.
amplifier:
You write on the schematic that the HP-coupling is useless due to
the long time constant. That is not true.
With 1meg and 1uF you get T=1 sec or fc=0.16 Hz and not 0.016 Hz.
Regards,
Joerg
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