From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2001-12-21 19:30:15
Dear Andreas:
> You are using 47pF caps in a couple of places. They are there to
> limit the bandwidth of the amps, right?
No these are "real world" devices to stop the amplifiers from oscillating. Most people omit these and wonder why they have intermittent troubles. Notice the last stage having to drive a .01 mf cap and cable. It may work without the 100 ohm and 47 pf and sometimes it may not. Unless you put a scope there you may never find the problem.
There is no high pass or low pass filtering. The 270 pf to ground is quiteimportant to keep the local radio station out of the measurements. Part of my biofeedback is a set of speakers that let you listen to the brain. That frequency response is out to 15 KHz. I wanted to hear everything. Whenyou chew food or frown, it makes an awful racket.
Just ahead of the A to D converter there is single high pass pole (one capacitor) and a capacitor to ground to help reduce aliases.
> Stupid question: Can the ESD-protection be improved if one puts a
> regular double diode to the shield/amplifier output so that the
> current can be diverted to the power rails as well?
Not stupid at all. I normally put diodes from the op amp output to the + and - rails and back to back diodes across the input. In this particular case I expect the output stage to handle the (hopefully minute) currents. The output stage is quite robust compared with the FET inputs. Since the amplifier is battery operated, there is no current path except for small capacitive charge/discharge. My purpose was to protect the input stage from feet on carpet type of ESD.
> Do you use a ground electrode as well, that is not shown in the
> schematic?
I use 6 connections to the head. F1, F2, O1,O2 and a ground electrode on each temple.
> What kind of problems did you run into before you added it?
> In what voltage range did these disturbances fall? Would an AC-
> coupled amplifier that can handle offsets of around 100 - 300 mV and
> transients of about 4 mV be sufficient?
Before I started I had the same bright ideas as everyone else on this list.I designed the perfect amplifier, etc. Then the real world brought me tomy knees. I know that you love your simulations. Unless you plug in the real world problems, your simulations will not be valid. If someone is strapped to a table and nothing in the room moves, and the connections are less than 10 K ohms, your simulations may be close. Just tap your foot on thecarpet and see what happens. One problem is that the skin is mainly a capacitor with some small resistance. It is about 0.1 mf with 10 K ohm resistance. The body can store 20,000 volts and you would not be aware of it.
So when you talk about a 4 mV transient, it is more like 4,000+ Volts. Howwill this charge be dissipated? The higher the input impedance, the longer it takes to reach a stable condition. I found very little difference in the EEG measurement by going to 10 Meg but the settling time and overload recovery improved vastly.
Peter I don't know whether you ever played with a record player? You may be too young. Anyway there is a shielded wire from the amplifier going up the tone arm to a cartridge that is either crystal or magnetic. If you put your finger on this wire going to the cartridge you will hear a hum so loudthat it might blow the speakers. If you ground yourself with the other hand the hum may get a little less. Playing the record showed not hum, but when you added your body, the "antenna" picked up the hum and sent it into the very sensitive phono preamp. Think of this analogy when you are workingwith brain waves. Many brain wave monitors solve this by providing a steep 60 Hz notch filter and using a sharp cutoff lowpass filter. During my research I wanted to know what the brain was doing in these frequency ranges,so that was not an option.
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: sleeper75se
To: buildcheapeeg_at_yahoogroups.com
Sent: Friday, December 21, 2001 12:51 PM
Subject: [buildcheapeeg] Re: My Input Stage
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
> Dear Group:
>
> I have hand drawn the input stage from the EEG that I built
> some years ago. Please look at my webpage under Brain Wave
> Monitor. EEG01.gif
Hi Jim, thanks for the schematic!
I've got a couple of questions:
You are using 47pF caps in a couple of places. They are there to
limit the bandwidth of the amps, right?
> If you look at the schematic you will see that this circuit is
> capable of more than 10 giga ohm input impedance, but ultimately I
> chose to use 10 meg ohms to ground. This is where the theory-
> simulation and practical collide.
>
> You will also see that this is capable to drive the shields and it
> works better with the shields grounded. Again practical.
What kind of problems did you run into before you added it?
Do you use a ground electrode as well, that is not shown in the
schematic?
> Note my clever guarded driven protective diodes.
Stupid question: Can the ESD-protection be improved if one puts a
regular double diode to the shield/amplifier output so that the
current can be diverted to the power rails as well?
> Note that I chose to DC couple the preamp.
>
> Note that I chose to run from +/- 12 volts and tried +/- 5 volts
> first. You need some common mode compliance to reduce overload
> saturation from real world events.
In what voltage range did these disturbances fall? Would an AC-
coupled amplifier that can handle offsets of around 100 - 300 mV and
transients of about 4 mV be sufficient?
/Andreas
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:35 BST