Var. skin imped. match

[Paul Lewis]

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.

https://upcommons.upc.edu/e-prints/bitstream/2117/1401/4/ROSELL-COLOMINAS.pdf

Can anyone suggest an appropriate circuit?

Paul Lewis
SCU

 

[Bob]

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.

https://upcommons.upc.edu/e-prints/bitstream/2117/1401/4/ROSELL-COLOM...

Can anyone suggest an appropriate circuit?

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

 

[John O'Flaherty]

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.

https://upcommons.upc.edu/e-prints/bitstream/2117/1401/4/ROSELL-COLOMINAS.pdf

Can anyone suggest an appropriate circuit?

Why do you need a "matching circuit"? What are you actually trying to
do?

 

[Fred Bloggs]

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.

https://upcommons.upc.edu/e-prints/bitstream/2117/1401/4/ROSELL-COLOMINAS.pdf

Can anyone suggest an appropriate circuit?

Paul Lewis
SCU

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=497047

"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..."

 

[Paul Lewis]

Why do you need a "matching circuit"? What are you actually trying to

do?

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

 

[Paul Lewis]

Can anyone suggest an appropriate circuit?

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=497047

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

 

[Bob]

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

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?

 

[John O'Flaherty]

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.

Use a constant current source (limited for safety, of course) and
forget about skin impedance.