Re: [Openeeg-list] Electrodes Polarity

 Re: [Openeeg-list] Electrodes Polarity From: - 2007-03-26 19:16:53 ```> Q: I better understood the inst-amp functionality. Something that > I want to be sure about is that for those reasons, inputs are noted > with a - and + polarities, not because a negative and a positive > voltage are feed from the sensors to the amp. > > Regards, > Brian Zammit This is correct. We are not measuring positive and negative voltages at individual elecrodes, we are measuring the voltage difference between two electrodes. So if the voltage at the + input was larger than the voltage at the - input, the output would be positive (with respect to the reference on the instrumentation amplifier). Hope that wasn't too confusing. Scott O ```

 [Openeeg-list] Electrodes Polarity From: Brian - 2007-03-26 18:47:58 Attachments: Message as HTML ```Sorry for the previously forwarded mail; Mistake. Ref: Analogue Schematic Date: Thu, 22 Mar 2007 14:39:42 -0400 (EDT) Original Message; Each channel signals from two different sensors are being fed as a "-"negative and the other as a "+" positive input.Why are they in this polarity? Am I confusing any simple thing?My understanding is this; one channel contains two sensors. Their screening will be connected to VGND while the signal from the two sensors as as Input. Where the polarities enter? Regards, Brian Zammit Reply; Each channel measures the voltage difference between the two inputs. You will notice that in the small schematic diagram on the instrumentation amplifier (IC202 or IC203) that the positive electrode input eventually leads to the positive input of the final op-amp inside the instrumentation amplifier, while the negative electrode input eventually leads to the negative input of that same op-amp. So if the instr-amp has a gain of 12.2, and you put 1uV between the - and + electrode inputs, you could get 12.2uV out of the instr-amp, whereas if you reversed the electrodes on that same 1uV signal, you'd get -12.2uV. Scott O Dear Members, Ref: Scot O Many thanks for your detailed explanation! It is highly appreciated. Q: I better understood the inst-amp functionality. Something that I want to be sure about is that for those reasons, inputs are noted with a - and + polarities, not because a negative and a positive voltage are feed from the sensors to the amp. Regards, Brian Zammit ```
 Re: [Openeeg-list] Electrodes Polarity From: - 2007-03-26 19:16:53 ```> Q: I better understood the inst-amp functionality. Something that > I want to be sure about is that for those reasons, inputs are noted > with a - and + polarities, not because a negative and a positive > voltage are feed from the sensors to the amp. > > Regards, > Brian Zammit This is correct. We are not measuring positive and negative voltages at individual elecrodes, we are measuring the voltage difference between two electrodes. So if the voltage at the + input was larger than the voltage at the - input, the output would be positive (with respect to the reference on the instrumentation amplifier). Hope that wasn't too confusing. Scott O ```