Patient Isolation

[JCafone1]

I am currently working on a bio-impedance device which will measure a human patient's skin impedance. Lately, I have been stumped. I am trying to devise a way to ensure the patient's safety. I have been looking into isolation method through isolation transformers and opto-couplers. I have been having a hard time though locating a part that will allow me to pass sine wave signals at frequencies up to 1MHz. The project is set up so far as follows:

Function Generator > Current Limiter > Patient > Differential Amp. > Oscilloscope

I would like to float all the grounds of the components in touch with the patient. This includes the ground points of the differential amplifier. The currents I am working with are minimal due the the current limiter (800uA). This will be done using Current Limiting Diodes.

Any help would be much appreciated.

Thanks

 

[flat5]

Use a battery.

 

[MikeMl]

I cant see how measurement of skin resistance intrinsically requires 1MHz bandwidth?

Why don't you use an isolated instrumentation amplifier where the patient side is battery powered using the same battery as is used for the stimulus.

 

[JCafone1]

I have already thought about that.
I would like to use some isolation method to prevent against any voltage surges.

 

[MikeMl]

I have already thought about that.
I would like to use some isolation method to prevent against any voltage surges.

I don't understand. Patient isolation requires isolation from the AC power lines used to power your oscilloscope, etc. There can be no Ohmic connection between the patient side of your circuit and the side connected to the 'scope.
The isolated amplifier I referred you to is rated at ±1500Vrms isolation between the input side and the output side. How much isolation do you think you need?

 

[JCafone1]

I just realized that the lab I am working in has an isolation transformer power station. Therefore, if I were to plug the output and input of the circuit into this power jack, does it provide isolation to the load? I will power the rest of the equipment using batteries. The circuit would be as follows:

Power>Isolation Transformer>Function Generator>Current Limiter>Electrode In>Patient>Electrode Out>Instrumentation Amp>Oscilloscope>Isolation Transformer>Power

Essentially all items in bold would be referenced to a floating ground as compared to the input Function Generator and output Oscilloscope.

Is this valid? If not please explain. Thank you!

 

[MikeMl]

You need to talk to the patient safety folks in your lab. University Labs and Hospitals I have worked in typically had stringent safety protocols which were especially rigorous when it came to experimentation on healthy humans...

AFAICR, we were not allowed to have a DC path between any AC-line-powered equipment such as your Function Gen or Scope and the subject, even if the equipment was powered via an isolation transformer. About 35 years ago I built a stimulator used to stimulate the occipital cortex of human subjects; it was battery powered and optically-isolated. Perhaps, since your application is less invasive, your human-experimentation committee might agree that the isolation transformer is sufficient.