Folks, does anyone know what the impedance (resistance) is for large skin electrodes? Either per square inch or for typical apps like these
6" by 4" for lumbosacral stimulation:I assume each secion is 3" by 4". On my own skin using aluminum foil I see about 400ohms at a few kilohertz, so if paralleled I should get
200ohms. I need to get into the
Although the model does NOT include any piezo effects which can play haovc, I got a lot of information from this site: in Figure 7, contact impedance drops as a function of Z < 200 near 180Hz and Z < 100 near 400Hz keeps dropping as you go on up in freq somewhere I have a VERY accurate PSpice model for EKG contacts which does include the piezo generator, but may have lost it in the multiple head crashes. If you need more than this site's info, let me know.
I believe they meant kOhm. This application is in a way different league where we need somewhere I have a VERY accurate PSpice model for EKG contacts which
We can't use EKG electrodes at all, they are only good for amplifier pickup. What we need is stimulator electrodes. I have seen some listend as low as 8 ohms but they didn't have a fast enough shipment option.
-- Regards, Joerg http://www.analogconsultants.com/
.
I only transferred what I was given. BUT I know my epithelial is over
200k, which means if you can break through that, you've got really pretty low impedance. As I remember, something like a bag of salt water, what is that? 100-200 S/m>? depending on how drunk you are?Aren't strong magnetic fields a form of stimulus without the need for contact?
If I were you and needed conductive contacts NOW, I'd use copper tape
- large areas. And if that isn't good enough, salt water soak and an ACE bandage over the whole thing.
I've always looked for a reason to have a couple of cold ones on the job :-)
We need conductive contact that can function from roughly 1.5kHz to
10kHz. The changes in the AC current are demodulated, it's about sensing inside the body.
We've got a solution now, this one:
If I use both sides in parallel I should be there. On my own body this size (mimicked with aluminum foil) has worked quite well and got me way under 100ohms.
-- Regards, Joerg http://www.analogconsultants.com/
If you are using a two-terminal system then I can understand why you need such a low impedance - you want to swamp impedance variations of the external electrode so that you only see things close to the internal electrode.
However, you are likely to get much better results with two external electrodes, one delivering the stimulus current and the other as a reference point for the voltage measurement.
Al is not a good candidate for stable measurements at low frequencies as the surface oxide layer properties will vary according to past usage. Traditionally, Ag coated with AgCl provides a chemically reversible electrode with stable properties.
As I'm sure you know, electrode impedances are strongly dependent on current density, as is the conductivity of the skin.
John
Like half of a Kelvin-method, yes, that would improve things. For now it's ok though, all we want to know is changes with the heart beat. Since I am measuring current a slight variation in electrode impedance would be ok. The system can also compensate for that (it does not simply measure amplitude, it watches resonance effects) but there comes a limit where we can't get enough current in there any longer.
I believe the pad from 3M is coated.
Yes, and some of that doesn't work in our favor. But we can go to several kHz so the capacitance of the epidermis helps.
-- Regards, Joerg http://www.analogconsultants.com/
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