ConMed ExcaliburPLUS PC - Page 41

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biased so that R9 is essentially in parallel with
R26. Then the incremental gain is -(R32/R26
+R32/R9). This steeper gain allows VBASE to
reach the voltage levels necessary for the higher
power settings. The voltage where the gain slope
changes is determined by the Thevenin eguivalent
of +12V, R11, R10 (shown as Vth and Rth in
Fig 3.2) and R9. R23 is summed into the break-
point amplifier to subtract out the effects of the
5V on the non-inverting pin of the amplifier, thus
allowing the output of the base voltage generator
to go to zero.
R34 and R35 form a voltage divider to generate
a 5V offset for U21-3 to allow single supply
operation and to give a reference point for the
VSENSE and ISENSE voltages. When RFEN is
low, U23-2 forces that voltage divider low, which
causes VBASE to turn off. When /WDTFL goes
low, U23-1 forces the base of A8Q1 low which
causes VBASE to turn off.
Vout
—
Vdac >
Vbreak=-(R9/Rt)*(Vt-5)+5-Vd
Figure 3.1 Base Voltage
Generator Transfer Curve
VoAc 862
no
20.0K
vm ATH DA ps
TT inatas
Figure 3.2 Basic Inverting
Break Point Amplifier
3.3.6 IFAIL DAC
Schematic 4.4b contains the IFAIL DAC A3U19.
The output of this 8 bit DAC (2.55 Vdc maxi-
mum output voltage) is fed into the reference
input of U23-9. The DAC is controlled by the
microprocessor to produce a reference voltage for
each mode of operation and power setting. The
ISENSE signal (refer to section 3.3.5 for a
description of ISENSE) is voltage divided and
lowpass filtered by R39, R49, and C61.
Whenever the resulting voltage exceeds the refer-
ence input voltage provided by the DAC U19,
AFAIL will go low and an interrupt is generated,
warning the microprocessor that the RF power
amplifier current is exceeding the maximum
allowed for that particular output mode and
power setting. This is a fatal alarm that produces
a “Err 5.2” code and shuts down the Excalibur
Plus PC™. Although it is possible for this to
occur because of a temporary fault condition and
may be recoverable by cycling power, it generally
occurs because of a problem in the Excalibur Plus
PC™ circuitry and should be checked by the
Biomedical Department before returning the unit
to service.
Note that the same DAC is used for the A.R.M.
circuitry approximately every 12 milliseconds for
about 200 microseconds. During that time, the
microprocessor ignores /IFAIL since the reference
voltage provided by the DAC is being used to
provide a signal for the ARMCOMP comparator.
See section 3.8 for a more detailed explanation of
the A.R.M. circuitry.
3.3.7 Waveform Generator
Schematic 4.4b contains A3U18, a 32Kx8
EPROM that stores the bit patterns for the wave-
forms that drive the RF power amplifier. The
EPROM is arranged so that the upper address
lines, WVO-WV7, determine which waveform is
selected by the microprocessor, and the lower
address lines, WAO-WAó6, determine which byte
of the waveform is selected at a time. The lower
address counter (A3U15 and A3U16) cycles
through its count to sequentially select each byte
of the waveform and to reload its own count
modulus at the beginning of each waveform. The
selected byte of the waveform is then parallel-
loaded into shift register A3U17, where it is seri-