Need precision about a schematic with LT1167 for EMG acquisition

Discussion in 'General Electronics Chat' started by chillyjee, Apr 26, 2016.

  1. chillyjee

    Thread Starter New Member

    Apr 26, 2016
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    Hi everyone, it's my first post here !

    I am trying to develop some 3 poles (+ , - , ref) electrodes with pre-amp stage for EMG signals acquisition. I was hesitating between AD8421 and LT1167 because both have very good performance for this application but I will choose LT1167 because of a higher input impedance (100 Gohm vs 30 Gohm for the AD) as I use gold plated electrodes.

    I was digging into the datasheet and found an example schematic entitled "nerve impulse amplifier" about indeed EMG acquisition :

    [​IMG]

    My question :
    I don't really understand this configuration with the patient ground (is it like a reference electrode ?), why is the Ref pin 5 connected to GND and not to the patient ground and thus why do they fix this electrode potential to the GND potential ? In the datasheet is said that the LT1112 OP Amp pin 1 creates a ground for the common-mode signal, I don't understand that. My (probably incorrect) thoughts are that the ref pin 5 should be connected to the body potential outside of the muscle activation zone to provide the common-mode signal ?

    As I want to use a reference electrode in my system sould I connect it directly to the pin 5 and forget about this patient ground part and not to the 0v GND ?

    Thanks in advance !
     
  2. #12

    Expert

    Nov 30, 2010
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    The patient ground is the reference electrode. It is placed rather far away from the EMG electrodes.
    The patient ground is part of a circuit that centers the input signals correctly by providing a centering signal for the LT1167 chip.
     
  3. chillyjee

    Thread Starter New Member

    Apr 26, 2016
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    Thank you for your answer !

    I understand that the patient ground is necessary to center the signal for the LT1167.
    The problem is that here it is that the ref pin 5 of the LT1167 is not connected to the patient ground electrode but to the ground. And the output pin 1 of the LT1112 is fixing the patient potential to 0V . Also isn't it dangerous to send current to the body ?
     
  4. #12

    Expert

    Nov 30, 2010
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    Why is that a problem?
    How does sending zero volts to the patient ground create dangerous current?
     
  5. chillyjee

    Thread Starter New Member

    Apr 26, 2016
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    Because if we use a ref electrode isn't to delete all the biopotentials that are noising the human body like ECG (that is reaching even the hands) or others that are not interesting in an EMG ?

    As the skin potential is rarely 0V, the op-amp has to flow current through the output to compensate in order to fix it to 0v and even uA on the long term can be harmful
     
  6. Normans

    New Member

    Jul 1, 2015
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    Does that mean that wearing clothes with all those uA of static electricity causes long term harm? I heard you can ward off such things with Al foil.
     
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