Details of operation
40
As mentioned above, common-mode rejection is a major
element in noise reduction (see page 36.) This is because many
forms of noise, mains interference in particular, are picked up
equally by both leads. Common-mode rejection is essentially a
DC phenomenon, but will apply to AC signals provided there is
no phase shift between the electrodes.
It is important to note, there must be two electrodes! The
common electrode should not be confused with a grounded
input. The differential inputs are high-impedance, so, if one
terminal is left unconnected it will float at an indeterminate
voltage. This configuration is highly susceptible to noise
pickup. The input will not somehow default to a single-ended
configuration.
To maximize AC common-mode rejection, phase shift must be
minimized. That is to say, the input time constant, C
n
R
n
36
, must
be minimized (see diagram on page .) Minimizing and
balancing the time constant by eliminating shared inputs has
been discussed above. Input capacitance can also be minimized,
using the Active Head Stage, 1902-10/2 and 1902-10/4 (see
page 3.) This is particularly useful if you are using mixed
electrode types: the capacitance of an Ag/AgCl surface-contact
electrode will be quite different from that of a skin-penetrating
needle electrode.
The Active Head Stage minimizes capacitance differences very
simply by reducing electrode lead lengths, thereby reducing
their capacitance. The 1902-10 is a battery-powered local
amplifier with low-impedance outputs which can satisfactorily
drive signals down long leads to the 1902. Being identical to
each other, these leads introduce no phase shift.
In order to be effective, the Active Head Stage has to be close
to the subject. Be careful not the let the box touch the subject
since this will increase noise pickup. The box should be
positioned out of the way, e.g. on the back of a headrest. The
1902-10 is internally shielded with a coat of conductive paint. It
MUST NOT BE EARTHED since this will compromise the
1902’s isolation.
Minimizing input
capacitance
Using the Active
Head Stage
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