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negative neutralization capacitance fed back to the probe input) will
result in a much faster risetime.
The neutralization control is preadjusted at the factory and in many
cases will never need additional attention. For very high
microelectrode resistances (above 10 megohms) or experimental
setups with a large amount of stray (or a co-axial cable)
capacitance, slight adjustments from time to time will be desirable.
Adjustments can be accomplished both when the 2400 is terminated
in (1) a microelectrode or in (2) a test resistor.
(1) It may be adjusted during an experiment by simply
introducing a square wave between the 2400 grounding lead
(shield of co-axial input cable or mounting rod or bracket) and
the experiment ground reference wire. Rotate the trimpot until
the leading edge of the square wave is vertical and displays no
more RC time constant (exponential) effects.
Actual signals may also be used if it is inconvenient to
introduce a square wave in the above manner. But because of
the lack of a clear rise time (or vertical leading edge) in most
situations the best approach will be to rotate the neutralization
trimpot until oscillations start, then back off one-half turn. This
should be very satisfactory in the majority of cases.
(2) The neutralization control may be adjusted to compensate
for fixed added input capacitance (such as a short length of co-
axial cable from the micro-electrode internal to the 2400 probe
input connector) in the following manner: Terminate the probe
input to ground through a 10 megohm resistor (carbon
composition type may be used). Now rotate the control until
oscillations are visible at the PREAMPLIFIER output BNC
connector. Back off one-half turn. This is also a good method
of calibrating the neutralization control if it is normally not
used but you desire optimum neutralization.
(b) PROBE INPUT PARAMETERS (PREAMPLIFIER MODE ONLY)
When operating in the PREAMPLIFIER mode, care should be
exercised with regard to static voltage discharge to the probe input.
Although the input is designed around a very rugged dual junction
FET amplifier and is isolated from the input through a 10K resistor,
extreme abuse (several hundred volts) may cause some damage.
The input stage is DC coupled to the microelectrode in the
PREAMPLIFIER mode (it is disconnected via a reed relay in all other
modes) and is operated at a gain of 10. For this reason, input DC
offsets should be less than one volt for linear recording. The use of a
