Details of operation
42
The timing unit can generate pulses in the range 0.5 to 14
milliseconds, the length being determined by the input source
selected by software. On receipt of the selected edge on the
Trigger 2 input, the timing circuit generates a pulse of the
selected length. The 1902 Mk IV has the option of selecting to
trigger on positive (rising) or negative (falling) edges of the
trigger signal.
The timed pulse operates switches that disconnect the electrode
inputs from the input amplifier. During the length of the timing
pulse, the 1902 output will be undefined, but will stay within its
linear range. At the end of the timing period, the input is
‘unclamped’ and the amplifier output will jump to a level
representing the new value of the inputs.
In addition to the four transducer input selections (Ground,
Normal differential, Reverse differential and Single-ended) and
the two isolated selections (Grounded EEG and Unclamped
EEG), there are thirteen additional isolated input settings. These
are given times in milliseconds:
0.5 ms, 1.0 ms, 1.5 ms, 2.0 ms, 3 ms, 4 ms,
5 ms, 6 ms, 7 ms, 8 ms, 10 ms, 12 ms, 14 ms
Other multiples of these basic times are available to special
order. So far as the user is concerned, these are normal EEG
inputs. However, selecting one of these timed inputs will arm
the clamp circuit, which will then clamp the inputs for the
selected length of time on receiving a trigger.
The clamp circuit begins to operate within a few tens of
microseconds of receiving a trigger, and the amplifier is fully
clamped after about 200 microseconds. The circuit can best deal
with large potentials on the electrode inputs if they do not arrive
before this time. The implication is that, if possible, the clamp
circuit should be triggered marginally before the stimulator.
One way of achieving this is to use external positive trigger
pulses about 0.5 to 1.0 milliseconds in duration, trigger the
1902 from the front (rising) edge and trigger the stimulator
from the trailing (falling) edge. The 1902 clamp time selected
should be 0.5 to 1.0 milliseconds longer to compensate.
Selection of
timing values
Connection of
external equipment
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