Multichannel Systems PGA Manual - Manual (Page 17)

Operating the PGA
13
4.3 Differential Amplifier and Noise Reduction
If the reference electrode is properly placed into an inactive region near the recording site,
the noise voltage is usually a common component of the input voltages from the reference
electrode and the recording electrodes. Thus, the noise will be canceled out when the difference
of the amplifier inputs is taken as the recording signal.
As a differential amplifier, the PGA has a reference input in addition to the ground input.
The power ground and the reference input are connected to the power ground and reference
output of the connected miniature preamplifier.
The reference input of the miniature preamplifier (in case that a differential preamplifier
is used) should be connected to a reference electrode that is ideally identical to the recording
electrodes and placed into a comparable but inactive area or tissue. This way, background or
noise signals that are picked up by both the reference electrode and the recording electrodes
are removed from the recording signal already in the miniature preamplifier. (For more details
on this issue, please see the MPA8I or MPA32I Manual.)
So what is the benefit of the differential operation of the PGA, if the noise is already canceled out
in the preamplifier? The differential operation of the PGA compensates the potential difference
between the miniature preamplifier output and the PGA input that results from the supply
voltage drop in the line from the preamplifier to the PGA. If multiple preamplifiers are connected
to a PGA, the amplifier offsets are averaged. Also, it is possible to connect a non-differential
preamplifier or the electrodes themselves directly to the PGA and cancel out the noise in the PGA.
However, connecting the electrodes directly to the PGA is not recommended, as the distance from
the electrodes to the first amplification stage should be kept as short as possible to optimize the
signal-to-noise ratio.