(3) U23 is a driver buffer for the energy indicators and the audible charging
indicator. U26 controls the logic for the error message. The error message is determined
by the external/internal paddle connector and the energy selector switch SW5. Err (error)
equals internal paddle set and energy selector levels of 100, 200, and 320.
r. Functions of the Circuits in the Paddle Preamplifier and Filter.
(1) Several important conditions have to be fulfilled by a paddle preamplifier
besides amplification of the signal. It has to be able to do the following:
(a)
Withstand high input voltages.
(b)
Have a very fast recovery time.
(c)
Have an excellent tolerance to direct current offset voltages.
(d) Have a very good common mode rejection, since there is no third
wire connection which acts as a reference.
NOTE:
In addition, it is usually advisable to provide heavy filtering out of unwanted
signals like 60Hz and any radio frequency interference.
(2) Despite the use of transfer switches in tandem with the fire buttons in each
paddle, enough high voltage transients may appear at the input of the amplifier to damage
it permanently. For this reason, two current limiting resistors (R168, R169) and two spark
gaps (K5, K6) clamp any high voltage transient to maximum 230v against the common
(ground) connection of the preamplifier.
(3) These transients are further current limited by resistors (R172 and R173)
to a few microamperes at the inputs of U45, a current low enough to be harmless to the
integrated circuit. Any permanent offset voltage produced by the polarization of the gel
used on the paddles is blocked by capacitors (C84, C85).
(4) The high input impedance in-series with each signal input requires that two
common mode adjustments have to be performed to reach a CMMR of 100dB or better.
The first adjustment takes care of the resistive component by varying, R18O. The second
adjustment C97, balances any phasing error.
(5) The first three sections of U45 form an instrumentation amplifier with very
little gain but with excellent CMMR. Whatever interference is left above the normal
frequency band of the desired signal is filtered out by a combination of notch and low-pass
filters.
MD0362
2-35
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