How to compare BioSemi specifications with the specifications of other systems? (Let BioSemi take you by the hand on a trip through specification-land)
 

Lets talk about our impedance and the 'impedance' of Competitor 'B'.
Impedance is the electrical resistance for alternating current. So, an impedance spec is only meaningful when given at a specific frequency. A high input impedance is certainly an important feature for an EEG amplifier, see for example this peer reviewed paper by one of the BioSemi founders. From this and other publications it can be learned that what matters in particular is the input impedance at frequencies within the EEG bandwidth. The input resistance (the impedance at DC) is relatively unimportant so long as it is high enough to prevent high input bias currents (and this is ensured by all contemporary FET and CMOS operational amplifiers anyway).

BioSemi fully specs the amplifier input characteristics with an input resistance of 1000 MOhm and an input capacitance of 11 pF. These values lead to an input impedance at 50 Hz of 300 MOhm (and the impedance at every other frequency can be readily calculated from the resistance and capacitance values). The exceptionally low input capacitance (which determines the input impedance at higher frequencies) could be achieved by using active electrodes: the amplifier input is as close as possible to the actual electrode, so stray capacitances by long wires and amplifier cabinets are avoided.

Competitor 'B' on the other hand, specifies only the relatively unimportant and easy to achieve "input resistance" (Input impedance for DC). The all important input capacitance and following impedance at frequencies within the EEG bandwidth are not specified at all. It basically leaves the potential customers in the dark about the abilities of the amplifier to handle electrodes with a high impedance (e.g. electrodes on the unprepared skin with impedances higher than 5 kOhm at 10 Hz), and the abilities to reject 50 Hz interference.

BioSemi specifies an input impedance of 300 MOhm @ 50 Hz (10^12 Ohm // 11 pF) (https://www.biosemi.com/activetwo_full_specs.htm)
Competitor 'B' specifies an input impedance (for DC) of > 1000 MOhm

So, the input impedance at DC is exactly the same (1000 MOhm = 10^12 Ohm), whereas the important impedance at 50 Hz is 300 MOhm for the BioSemi, determined by the ultralow 11 pF input capacitance (Competitor 'B' does not provide the input capacitance and related AC input impedance at all)

 

Lets talk about input noise specifications.
Willl be added shortly....