Measurement of Wire impedance

Discussion in 'General Electronics Chat' started by vijaybala85, May 22, 2012.

  1. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    Hello,

    This may be very basic question, but I would like to confirm my understanding because it has been a while.

    I have these set of ECG lead wires, whose resistance I am trying to measure because I am not able to find the specs yet. Can I just do that by using a multimeter at the 2 ends of the wire and read the value? Please let me know.
    Thanks!

    V
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    you can try to measure them with a multimeter. Unless they are ECG cables for MR. They will probably just be a metal wire. How accurate your data will be will depend much on the multimeter. But you will be able to detect a bad wire.
     
  3. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    Thanks! While I am waiting for the specs from the manufacturer, let me pose an even basic question.

    How is the ECG signal from the heart sent through the electrode, lead wire into equipment for processing and display?

    I imagine the underlying concept to be this way. because of the way I see how the leadwire looks like. I see a clip end that attaches to the electrode and the other end has 2 pins that attaches to the module.

    Is my understanding correct? I get into these underlying principles which bug me a lot :)

    When the leadwire is not attached to an electrode, essentially it is an open circuit? Attaching the clip onto an electrode completes the circuit. Thus, the electrical pulse from the heart is sent through this completed circuit into the module for processing. How is the small current recorded and what is its direction?

    I am trying to figure out the flow of signal and what completes the circuit. I hope my question is clear. Most documents I read talk about the leads and just say the signal is recorded. I am interested in how it is recorded. Again, these seem too trivial but it is bothering me :) Any help would be appreciated.
     
    Last edited: May 22, 2012
  4. t06afre

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    May 11, 2009
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    Hmm that was a drastic change in subject. Do you have some ECG project. And somewhat unsure what to do? Your question is to wide to answer by a post. But this link may help you on the way http://www.bem.fi/book/
     
  5. vijaybala85

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    Jan 7, 2010
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    The reason I was asking that is because I am wondering if I need to measure the impedance across the leadwire when it is connected to an electrode and the electrode is attached to the skin? Wondering across which two ends should I place the multimeter? Is the clip one end or is the clip like a switch that completes a circuit? Because the other side of the leadwire, I see two pins.

    Thank you for the reference. I understand the electrical flow in the heart. I am trying to understand how it is recorded? Is it just inducing an electrical current on the leadwire? and this brought me to my question in para 1.

    It can't be this hard wonder why I make it complicated :)
     
  6. crutschow

    Expert

    Mar 14, 2008
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    The EEG is a voltage that is transferred by the leadwires to the EEG amplifier's input terminals. These inputs have a high input impedance so only a tiny current (much less than a microamp) flows in the EEG leads, thus the resistance of a typical leadwire would have negligible (and unmeasurable) effect on the signal. So the impedance of the wires is of no concern when talking about the EEG signal.

    The amplifier then amplifies the small signal to a level (typically a few volts) that the following signal processing circuits can readily process for recording and/or display on a monitor.
     
  7. t06afre

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    And do not forget that Google is your friend. You will find many ECG projects on the web. Some are bad others are good. Many big component manufacturer like Analog Devices, Texas Instruments. Have very good application notes on this topic
     
  8. vijaybala85

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    Jan 7, 2010
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    Hmmm.... If only a small current is recorded, wouldn't the leadwire have high impedance? And knowing this impedance is essential to a particular test we are trying to perform to ensure it doesnot cause any heating effect on a person when in an MRI. The levels of current that can be induced if the leadwire in a high RF field are quite high if the impedance is low.
     
  9. vijaybala85

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    Jan 7, 2010
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    These videos help a lot in understanding ECG for anyone interested
    http://www.youtube.com/watch?v=z1Emrz-9Vuc&feature=relmfu

    Gives an engineering perspective along with clinical. Now that I am caught up, I am still trying to figure out which two ends of the lead wire should be connected to my multimeter.. hmmmm
     
  10. t06afre

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    I did mention the MR problem in my first answer. Why are you just letting out information in small dripping. Trying to help you in any manner then you communicate in this way is just futile
     
  11. vijaybala85

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    Jan 7, 2010
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    Hi t06afre... I am sorry.. I didn't think MR would be an issue with respect to measure the impedance of the wire. Basically, I am trying to determine if the cable is used in an MR field would it have any effect. Hence, I need to determine the impedance specs.
     
  12. #12

    Expert

    Nov 30, 2010
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    A typical lead wire has very close to zero ohms. The way to limit induced current from the magnetic field is to connect it to an amplifer with high input impedance. This method makes the resistance of the lead wire irrelevant. You are truly over complicating the subject.
     
  13. t06afre

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    Using metal cables inside a MRI bore has caused several accidents and burns. It does not help help using a high impedance amplifier. The only way is to use high impedance cables. In an effort to minimize distortion and artifacts on magnetic resonance images. Radiotranslucent materials are used in the ECG electrode leads. Carbon fiber leads or leads made of graphite are common. Carbon fiber leads reinforced with plastic. Are preferred as they are less vulnerable to bending than leads made of graphite.
     
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  14. #12

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    Apparently I missed on this one. I didn't think the magnetic fields could close a current loop in the length of a lead wire.
     
  15. vijaybala85

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    Jan 7, 2010
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    My conundrum is with this. The lead wire needs to be of zero or less resistance to read low voltage ECG signal from the heart to be transmitted to the module for processing

    But, low resistance/impedance lead wire is susceptible to high currents being induced during high RF fields of the MRI or even when using a defibrillator.

    When I measured between the two ends of the Radiotranslucent lead wire that I have (good guess on that t06 :) ) I see a value of 250 ohms which kinda confirms what #12 says, but again, I am not sure if I am measuring it right.

    Again, it may be right because I don't know if they can be used in an MRI's field so they are not made as high impedance wires. Hence my conundrum :(

    I read somewhere that ECG lead wires for MRI have high impedance insulation. Is there something to that?

    Thanks!

    V
     
  16. #12

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    Nov 30, 2010
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    Ah! There's my mistake. I didn't know an MRI had RF fields.

    By the way, the definition of insulation is that it has high impedance. There is nothing special about ECG leads having high impedance insulation.

    and there is no such thing as "less than zero" resistance in a wire.
     
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    MRI does make use of RF signals.
    The frequency of the signals are dependend of the strength of the magnet.
    A 3 Tesla magnet will have a Proton frequency of about 127 Mhz.
    For more information you can have a look at the wiki page on MRI:
    http://en.wikipedia.org/wiki/MRI

    Bertus
     
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  18. JMW

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    Nov 21, 2011
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    DO NOT do this. Medical equipment is not to be messed with. All parts used in medical equipment have to be certified by the manufacturer as to there suitability. If you read the fine print on the back of IC's etc you will see this disclaimer.
    That being said, the voltage you will see is approx .001 volt across the leads. Op amps and opto isolators are used to isolate the person from the wall plug. This is interesting as the ECG/EKG readings were taken in the early 1900's before vacuum tube amplifiers. I think EEG 1 microvolt range were also observed.
     
  19. tpny

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    May 6, 2012
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    Isn't the resistance of any wire effectively 0? It's a conductor. You connect it between 2 points and it's a short circuit.
     
  20. t06afre

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    May 11, 2009
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    This is not true. If you look at any PROPER ECG schematic you will see safety resistors equal to several hundreds Kilo ohms placed in series with the ECG lead. A few extra kilo ohms in the leads. Will not have anything to say. This because high input impedance amplifiers with input impedance in the range of tens of Giga ohm to Tera ohm range. Are used in the ECG frontend. And they have to be, else you would just "short circuit" the ECG signal source origin and read nothing.
     
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