I refer to figure 4 in the interesting paper below. It is a chart showing that skin impedance varies from 100 Ohms to 100K Ohms across the 1MHz to 100Hz range.
I need to build a variable impedance matching circuit for micro-current skin electrodes that covers this full range. I am a graduate student in biophysics, but not an electronics engineer.
Can anyone suggest an appropriate circuit?
Paul Lewis SCU
You say microcurrent which suggests to me that you are trying to measure currents generated by biological processes in the body. Is this correct?
The study you link to is of the skin impedance when applying a current to the body from an external energy source.
I doubt that the source impedance of AC signals generated by the body is anywhere near the same as the load impedance seen when applying external current.
Externally applied currents will flow through whatever conductive paths are available between the electrodes, mostly through the watery bits I'd guess.
A body generated voltage observed at the skin will have a source impedance due to the impedance of whatever chemical process is generating it deep inside the body in a cell or muscle, the conductive path from whatever is producing the signal and the shuting effect of current flowing through other conductive bits of the body instead of out of one electrode and back in the other.
I think it is unlikely that body generated currents will have a source impedance as low as 100ohms at external electrodes.
Bob
Why do you need a "matching circuit"? What are you actually trying to do?
-- John
"Electrodes with impedance matching at the sensing site are referred to as active electrodes and have been designed since 1960's [15-17]. The electronic part of these transducers mostly consists of a buffer amplifier, but some have been designed to need only two lead connection wire [18,19]. However, as the signal is not amplified, buffers introduce significant noise and a low noise amplifier is still needed at the front-end. In order to avoid this drawback we used a two-op-amp biopotential amplifier [20] shown in Fig. 3, where op-amps A0 and A1 were integrated at the electrodes (Fig. 4), instead of using extra buffers. This resulted in lower noise and less parts, at the expense of increased number of electrode leads. The amplifier is based on the two-op-amp instrumentation amplifier shown in Fig. 5. The output voltage of the basic two-op-amp amplifier is..."
It is a system that applies low voltage microcurrent pulses to the skin to alleviate pain. We are presently in research and trialling a wide range of frequencies, eg. the 100Hz to 100KHz previously mentioned. Hence the need for an adjustable matching network. This can be either switched or continuous adjustment, via a pot.
Any circuit suggestions would be most appreciated.
Paul Lewis
That's a brilliant design, but would be a whole research project in itself to replicate.
I was hoping for something with adjustable passive components. Is there a viable approach to this?
Many thanks,
Paul Lewis SCU
Define "microcurrent"
If you actually mean microamps then you don't need a matching circuit. 20volts into 100Kohms will give you 200uA I doubt you will be using lads of more than a couple of meters so transmission line effects will not be significant.
Worrying about output amplifier dissipation at microamp currents is unlikely to be worth it. Are you trying to get this to run for six months on an AA battery or somthing?
You mention 100hz to 1MHz in your first post and 100Hz to 100KHz above. Which is it?
Use a constant current source (limited for safety, of course) and forget about skin impedance.
-- John
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