How to perform 200kHz-5MHz sweep while maintaining 200mArms?

[Michael]

I am performing a test to measure the impedance through electrode pads
placed on opposite parts of the human body. The protocol requires that
the measurement be made by maintaining an AC sine current of 200mArms,
sweeping the frequencies from 200kHz to 5MHz, and measuring the Vrms
between one electrode and a reference point on the body. The impedance
is to be calculated by dividing this voltage by the known 200mArms
current.

How do I sweep frequencies while maintaining a constant 200mArms
current using a HP 33120A (waveform generator w/ 0-15MHz freq and
0-10V peak to peak) or a HP 4192A (impedance analyzer w/ 0-15MHz freq
and -1V-+1V peak to peak)? Can I construct a circuit external to one
of these units and accomplish the 200mArms sweep? If not, what
lowest-cost set-up could?

 

[Walter Harley]

"Michael" <maxel@berkeley.edu> wrote in message
news:ce5951b4.0409221342.44bb1c8b@posting.google.com...

I am performing a test to measure the impedance through electrode pads
placed on opposite parts of the human body. The protocol requires that
the measurement be made by maintaining an AC sine current of 200mArms,
sweeping the frequencies from 200kHz to 5MHz, and measuring the Vrms
between one electrode and a reference point on the body. The impedance
is to be calculated by dividing this voltage by the known 200mArms
current.

How do I sweep frequencies while maintaining a constant 200mArms
current using a HP 33120A (waveform generator w/ 0-15MHz freq and
0-10V peak to peak) or a HP 4192A (impedance analyzer w/ 0-15MHz freq
and -1V-+1V peak to peak)? Can I construct a circuit external to one
of these units and accomplish the 200mArms sweep? If not, what
lowest-cost set-up could?

There are some questions which should not be answered, because answering
them just increases the risk of harm. This is one of them.

It is true that high frequencies reduce the risk of electrocution, because
of the "skin effect" (which has nothing to do with your skin, it applies to
wires also). (This effect also means that if you measure with those
frequencies, you're not measuring anything much to do with the human body
per se.)

But it is also true that any time you get near a human body with an
electrode and a power source, there is a risk of electrocution. 200mA at
lower frequencies will definitely kill, and if a fault occurs in your
equipment the subject could be exposed to arbitrary frequencies and
currents. You better believe HP didn't design that gear with the intent of
connecting it to a person.

You need two things, if you're going to do this in an educational/research
environment:

1. Contact with, and permission from, your human studies review board.

2. A medically approved stimulus apparatus (which will contain certain
safety mechanisms that guarantee that the stimulus is non-lethal).

 

maxel@berkeley.edu (Michael) wrote:

I am performing a test to measure the impedance through electrode pads
placed on opposite parts of the human body. The protocol requires that
the measurement be made by maintaining an AC sine current of 200mArms,

You better check that spec again, two hundred milliamps? Diathermy
seems to happen at 500 mA... Maybe you mean microamps. Please don't
kill anybody getting this wrong!

http://www.rjlsystems.com/research/bia-principles.html