From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-11 08:39:52
Dear Joerg:
>> What do you think of the TMA0505S DCDC converter ? It has
>> low coupling C and high isolation R)
>I don't know anything about this DC/DC converter. Would you give me a
>reference to your schematic
I looked at your schematic and saw how you used it. My opinion is still the same. The patient's safety depends on that cute little transformer. BTWMy company manufactured transformers for a few years. I know how they canfail.
> Better not. I do not like batteries. They are always empty when I need them,
> have memory effect (NiCad), self discharge, need much volume, have greatweight (compared to a little DCDC
> converter).
I like batteries ( they are like women, you have to treat them right ).
I have a compromise proposal. If the source that drives the DC/DC converter were to be a 6 or 12 volt battery, it would be safe. Then the opto isolators are the weak link.
So "you" can use an AC power supply to drive the DC/DC converter.
Why not give others the option to use batteries.
(BTW) I noticed the filter in front of the A/D converter. Do you really mean a 20 K ohm and a 100 nf cap. What frequency does that roll off at?
Is this board a working design? Have you actually built these?
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: Thursday, January 10, 2002 5:38 PM
Subject: Re: [buildcheapeeg]Patient Safety and Input protection
Dear Jim:
----- Original Message -----
From: Jim Meissner <jpmeissner_at_mindspring.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Tuesday, January 08, 2002 3:56 AM
Subject: Re: [buildcheapeeg]Patient Safety and Input protection
...
>> What do you think of the TMA0505S DCDC converter ? It has
>> low coupling C and high isolation R)
>I don't know anything about this DC/DC converter. Would you give me a
>reference to your schematic
http://www.jhansmann.de/eeg/modEEGdigital01.pdf
> so I can look where you are using it. I would not trust it for patient
> safety isolation no matter what the spec said. Opto isolation is safer. ( in
> my opinion )
> my website if you are interested in "my" solution. = "Iso01.gif" )
I have downloaded it.
>> Can it handle speeds of 56 kbaud ?
> All I needed was 9600, never tried higher.
> you can use Brian's circuit using QSE158-ND ($1.42, qty
> 1)and QEE113-ND ($.74, qty 1) with 1 or 2 inch spacing between the
> LED and photodetector on separate PC boards. ( I really like
> this. )
>> Can it handle 56 kbaud too ?
> We need to ask Brian. The T/on T/off is 100 nsec. for the receiver. What
> do you think?
If the transmitter is as fast as the receiver it can handle even 112000baud
easily.
> A floating ground requires some expert consideration of impedances at various
> frequencies.
>
>> The ground (exactly Vref/2) of the modularEEG is not "floating".
>> It is connected to the low-impedance output of an op-amp and
>> heavily bypassed to +5V and 0V.
>
>The low-impedance of the op amp is "not" very low!
Lets assume an open loop output impedance of the Vref/2 buffering OP
of 50 Ohms and an open loop gain of 100000, then for low frequencies
the output impedance will be 50 Ohms / 100000 = 0.5 mOhms.
For higher frequencies the bypassing Cs hit in.
>Any "real" capacitor is
>only a low impedance over a relatively small frequency range. Electrolytic is
>good at low frequencies, then you need a solid tantalum,
For low frequencies the closed loop output impedance should be good enough.
>then you need a
>monolithic ceramic, etc. I would like to see your simulation
For what circuit exactly ?
(BTW. I have done a simulation for the current TinyEEG analog
power filtering - (in another posting) )
> using "real"
> capacitor impedances over frequency with Xc series R series L. I have had to
> fight this grounding battle in the past. It will never be as good a ground
> as the chassis or PC board ground plane.
For HF ceramic Cs at each IC should be sufficient.
For low-frequencies I would rely more on the low closed loop
output impedance of a regulator or buffer.
No one prevents you from defining the buffered Vref/2 as the ground plane.
In fact I have done that in the modularEEGdigital pcb by copper pouring.
With a 2-layer pcb I would use a real separate Vref/2 plane below the analog circuitry.
>Something always oscillates. This
>would be a good application of a DC/DC converter to generate a +/- buss for
>the op amps. A simple clean solution. You may even need a separate power
>supply or batteries to decouple the input stage.
Better not. I do not like batteries. They are always empty when I need them,
have memory effect (NiCad), self discharge, need much volume, have great weight (compared to a little DCDC converter).
Regards,
Joerg
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST