Welch Allyn PneumoCheck 61000, Service Manual

Page: 28 / 90

Service Manual 610132 Rev. E Pneumocheck Handle Printer/Charger 27
TP2 and TP3 should m/- 3.90 to 4.10 volts. These
voltages power the pressure transducer as well a U10 to
U11.
The switched analog voltage is also, regulated by U15B
and T2 to give an isolated 5v +/- 0.10 volts at TP6. This
voltage powers the A/D and U12. Under the assumption
that both devices will never be used at the same instant,
this power is also used to supply U13 on units with a
610117-501 pressure transducers. Units with a
6010116-501 use Vcc.
Since the reference voltage from the A/D and the
reference voltage of the transducer are both derived
from Vcc, any variations in Vcc are ratiometrically
canceled out.
U10 and U11 are powered by VA+ and VA- because
they must deal with negative voltages. However, since
VA+ is less than Vcc, they cannot swing the full voltage
range of the A/D. Therefore, U15, the first amplifier
stage, is powered by V+.
When pressure sensing, VA+ and VA- must be on. The
output of the transducer is a small differential signal
(0.45 to 0.9 mv/cm H
2
O) on a common mode signal of
about -1 volts.
U10 forms a differential amplifier with adjustable gain.
U11B removes the common mode voltage, provides an
appropriate level shift, and provides a further gain of 5x.
The output of U11B is frequency limited to a few
hundred hertz by C25.
The bais voltage is supplied by a software charge
pump. When appropriate software is running 0-1 Khz
pulse train from pin 16 of the CPU charges C2 to 0-2
volts (TP4). U11A buffers this voltage and uses it to
offset U11B.
4. Spirometer Analog
Circuit Description
Section 3 - Theory of Operation