During the early nineties, researchers kept asking for systems with higher dynamic range (to allow DC measurements), better channel separation, and more channels than could practically be achieved with the Shared-ADC-setup. Advancing developments in low-power, high-bit ADCs and low-power Programmable Logic Devices (PLD) allowed designers of modern multichannel acquisition systems to switch to the ADC-per-channel setup. BioSemi already introduced the new setup on the Mark-6 system in 1995, and has since then continued to improve the setup in terms of miniaturization, power consumption and reduced costs. The ADCs operate synchronously, so no sampling skew is present (all ADCs perform the conversion at the same moment). The multiplexing operation is now performed entirely digital, so any deterioration of the signal is eliminated. The dynamic range of the system can now be really equal to the dynamic range of the ADC. The 24 bit ActiveTwo with its 110 dB dynamic range and >110 dB channel separation is a good example of the advantages which can be achieved by using one ADC-per-channel. Specs like these are fully unattainable with the older setup (30-40 dB worse, a factor of nearly 100).
The important step forward that could be achieved by using an ADC-per-channel and digital multiplexing, forced all serious manufacturers to use this setup for their new designs. Consequently, you will only find the Shared-ADC-setup in older designs.
Customers not acquainted with the performance features of modern ADCs, sometimes raise the question whether the new setup does not introduce additional errors as a result of differences between the ADCs. As they see it, the use of a Shared-ADC-setup at least ensures hat any ADC error is the same for all channels, and thus cancels when the difference between channels is displayed. Although this assumption is not untrue, these customers ignore that the differences between modern ADCs in a properly designed circuit topology are much smaller than the errors introduced by the analog multiplexing circuitry.
A lot of work has gone into optimizing the circuitry used in our systems in order to minimize channel differences. One of the key design features is that we use a Zero reference setup with a common Reference voltage for all (up to 256) ADCs. The circuit board has been optimized (regardless of costs) for identical REF voltage for all ADCs. For example: REF is distributed among the ADCs by low-impedance, low inductance ground planes, a method effectively preventing small voltage drops across the motherboard. Consequently, this is one of the reasons why we insist on having all modules on one single motherboard. You can never attain this kind of precision when you start coupling subsections with for example 32 channels. Another feature of the "Zero reference setup" is that we make a final subtraction of the channels in software, effectively canceling all noise and drift effects of the ADC references. Finally, all the ADCs run on the same master clock to prevent any timing differences between the ADCs (which may effect the conversion).
In a multichannel system, there are always small gain differences between the channels. Part of these differences are caused by resistor tolerances in the amplifier stage between active electrode and ADC, and the other part is caused by tolerances in the analog sections of the ADCs. The difference between the ADCs is much less than 1% (guaranteed by the manufacturer, and checked by us). We found that the dominating source of gain errors between the channels are resistor tolerances in the amplifiers. BioSemi exclusively uses precision metal film resistors to ensure high accuracy and stability over time. We specify an overall gain accuracy of 1% (both absolute and relative). Further software calibration can be added for anyone insisting on better accuracy. It should be noted that the amplifier gain tolerance problem is exactly the same when using a Shared-ADC-setup scanning the channels.
Given the excellent accuracy of modern ADCs, and the way they can be made to perform identical in a properly designed system, the ADC-per-channel setup actually makes it far more easier to ensure equality of the channels than the obsolete Shared-ADC-setup