-----Original Message----- From: Caisson To: PICLIST@MITVMA.MIT.EDU Date: Friday, January 14, 2000 3:41 AM Subject: Re: [OT] Differential inputs (was how to.. (was :Handicap Mouse & Computer Interface)) >> Van: Thomas McGahee >> Aan: PICLIST@MITVMA.MIT.EDU >> Onderwerp: Re: [OT] how to.. (was :Handicap Mouse & Computer Interface) >> Datum: donderdag 13 januari 2000 15:42 > >Hello Thomas, > > Could you explain to me why you position the Ground-wire between the >differential inputs ? I would think that those should be as close together >as possible (to get the same effect of outside disturbances on both lines), >wedged between two Ground-lines. > >Most differential inputs cables I've seen where either Coax (two wires with >a Grounded shielding), or a simple twisted three-wire cable. > >Regards, > Rudy Wieser It is mostly a matter of convenience. The three wires are twisted together to reduce common mode noise. The "ground" electrode then goes in the center to maintain electrical symmetry. You can also surround the two signal wires with a ground, but this is a bit messier to implement in the real world. You can obviously use coaxial wiring instead of the twisted 3 pair wire, but for the target application you do not really gain much for the extra expense. Remember that the target application is simply to get a *useable* signal from a muscle movement. If we were interested in the exact waveform (as in electrocardiographs), then the extra expense is warranted. In regard to the spacing between the active electrodes, it depends on the signal source. For the heart you would want them separated by at least a few cm since the heart is located rather deep. For muscles located nearer the surface you can reduce the spacing. Note that if separation of the electrodes is too far, then you get a mixture of several muscle signals, and that is fairly useless. For the target application (handicapped person), you would want to find a muscle that the subject has good control over, and then isolate that signal as much as possible. One subject might use a finger muscle, and another a jaw muscle. It also helps a lot in the real world if you can make an electrode carrier that can be readily applied and which is comfortable to the user. Fr. Tom McGahee > > >> The electrode interface begins with an isolated instrumentation >> amplifier with two "active" differential inputs and a third >> "guard" electrode that establishes the ground reference. >> >> The "guard" electrode is physically located between the two >> differential inputs. This helps to cancel out much of the >> common mode signal. This helps to cancel out much of the >> common mode signal. > > >