# Help with circuit design

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I wish to design a simple circuit, but I do not have the knowledge required. So I was hoping someone could help me. I am willing to pay a modest fee for your time.

Here is a description of the function... I wish to measure the change in potential across the human body. The baseline potential is about 70mV and the change is around +/- 30mV. A basic indication is all that I require. The function is to calibrate the device to get the potential at this moment by turning a dial to make two LEDs go off. Then the stimulus is applied to change the potential. If the potential goes above the calibrated set point, one LED comes on. If is goes below the calibrated set point, the other LED comes on.

Thanks Damien

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Dual comparator, pot, LEDs, very simple. Why do it this way when you can buy a \$5 DMM that reads the potential value directly, with no LEDs.

NT

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Thanks. Been experimenting with a DMM. Now looking to go simpler and portable.

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This is a high input impedance circuit. iows an antenna. What about emi pickup?

IF I had to work on this it I would begin with playing with the scope. I would experiment by attaching each probe across the body. Then I'd check Probe A and Probe B voltage difference. Then set filters. This would give me hints on emi problems.

Yeah the digital part is easy. The analog front end could be a bitch.

Add on active/passive filters + common mode amplifier?

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Oppss.. Not common mode amplifier... Differential amplifier. I was thinking common mode rejection.

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Please note that in a normal residential / office environment, a person has about 200 pF to the ground (supply neutral line) and about 20 pF to the active phase line of the 115 / 230 V supply to the building. This creates a high-impedance voltage divider floating the person at 12 to 23 V 50/60 Hz. On top of this come all RF / EMI sources around.

A simple measurement system may get incorrect indications from the stray voltages.

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Tauno Voipio```
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A DMM might be sampling at some subharmonic of the 50 and/or 69 Hz, such as 10 Hz (100 ms sample period), thus the mains hum (and harmonics) are effectively integrated out and there are deep notches at these frequencies, creating a comb filter. If the mains frequency deviates a lot from the nominal 50/60 Hz, phase locking the sample period to the actual mains frequency might help.

Of course the RFI must be filtered before the amplifier, but this should be quite easy, since the corner frequency can be set well above the typical bioelectric frequencies.

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Sign up on elance, and do a proper bid and job... ;-)

Charlie

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Sounds like he needs a human Faraday cage,

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I stumbled across your request, I have a suggestion:

It looks like you could use (for measuring voltages across the human body)

a specialized instrumentation type amplifier, like those used on Holter heart monitors.

One place you can see application notes and circuits (for this application) is Analog Devices

, one product that may be useful is AD8235,

but there are many others.

Some keywords you can use is "biomedical amplifiers", "ECG monitor", "cardiac monitor", to use in narrowing your search, when looking for information on that web-site. Their products are second-to-none!

Hope this helps,

Doug Maenpaa

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