Let me explain by starting with the most simple setup, and then making thing more complicated step-by step until we end up with the actual ActiveTwo configuration. The initial goal is to measure the potential difference (voltage) between two electrodes.
1) The most simple setup is to use one Ground (GND) electrode, and one measuring electrode (E1). Electrode E1 is connected to the high-impedance input of an amplifier, the GND electrode is connected to the 0V of the amp. The amplifier amplifier E1 with respect to GND, the output signal is digitized and displayed.
This setup does not work very well because interference currents are flowing via the GND electrode (see
http://www.biosemi.com/publications/pdf ... uction.pdf). Current through the GND impedance causes a voltage. This interference voltage is by definition called the Common Mode voltage. The CM voltage is added directly to the EEG signal, the Common Mode Rejection Ratio (CMRR) is zero. Even with an optimally isolated front-end (battery power, fiber coupling), the CM voltage will completely mask the EEG in this situation.
2) To improve thing, we replace the simple GND connection with a driven ground electrode circuit (Driven Right Leg). As the input of the DRL integrator we use an extra CMS (Common Mode Sense ) electrode. The GND electrode (called DRL from now on) is within a feedback loop, and its effective impedance is divided by the loop gain. Consequently, the CM voltage is reduced by the open-loop gain of the DRL circuit. The amount of open-loop gain is limited for stability reasons. Still, an open-loop gain of 100 at 50 Hz is easily attained. So, the CM voltage at 50 Hz is reduced by a factor of 100 (40 dB) with respect to situation 1. In other words, we have improved the CMRR to 40 dB.
In spite of this improvement, there are two remaining problems with this setup. First, 40 dB CMRR is not enough in most situations. Secondly, the location of the GND potential becomes dependent on the frequency. The reason is that the open-loop gain of the DRL driver decreases with frequency, So, for low frequencies (where open loop is gain is high) E1 is measured with respect to the CMS electrode, whereas for higher frequencies (where the open-loop gain is low), E1 is measured with respect to a location somewhere between CMS and DRL electrode (wandering toward the DRL for increasing frequencies).
3) To solve the two indicated problems, we now add a second measuring electrode E2 and corresponding amplifier channel. Instead of looking at the difference between E1 and GND/CMS/DRL as in situations 1 and 2, we look at the difference between E1 and E2. The difference between E1 and E2 can be calculated by analog electronics (use an instrumentation amplifier), or with digital electronics (subtract digital output words of analog-to digital converters).
This new setup solves the problems still present in situation 2. The CMRR is increased by a large extra factor depending only on the matching of the channels (40 dB extra in case of ActiveTwo). Also, the location problem of the GND point (somewhere between CMS and DRL) is eliminated: E1 and E are both measured with respect to the same point, we only look at the difference between E1 and E2, so the exact location of the GND point doesn't matter anymore.
Finally, I'll comment on the to suggestions for modifications of the setup given in the second post
- Why not use the output of the CMS signal as the actual reference ?
This is indeed an alternative (analog) method for increasing the CMRR. The problem with this method is that in a multichannel system, the CMS buffer will have to drive many parallel instrumentation amplifier inputs (up to 256 in our case). This proved to lead to several stability problems.
- Why not thread the CMS as just another channel ?
This is the same as asking: why not us one of the measuring electrodes as CMS signal?. This is indeed possible and would spare an electrode (no separate CMS anymore). For example: E1 could be use both as channel 1, and as input for the DRL integrator. There are two reason why we prefer an extra separate CMS. First, the concept of having one special channel that always have to be connected is confusing (for example: several users do not use channel 1, or do not always use channel 1). Secondly, an important design principle in the ActiveTwo is optimal equality of the channels. With this suggestion, one channel would be very slight different form the others (one active electrode has to drive both the ADC and the DRL integrator, all others only drive the ADC, load by stray capacitances would be slightly different). This would jeopardize the perfect symmetry that we want to achieve in the design.
Best regards, Coen (BioSemi)