BCI Capnocheck Plus - Pressure Sensot; Battery Voltage Level; Fio,Sensor

Chapter 3: Capnocheck Plus Circuit Description
The voltage level from the Bench, which indicates CO; concentration, comes
to the gain-offset stage built around U15A and R48. It converts the Bench
voltage level into an ADC compatible voltage range. The gain - offset equation
is:
Vout = 2.77 * Vin - 1.646V
Another function of U15A in combination with R82, R81, C89 and C88 is to
create a low pass filter to reduce out-of-range noise.
A voltage level proportional to the CO: detector’s temperature comes directly
to the input of ADC. It does not require any amplification or level shifting.
Pressure Sensor.
The Pressure Sensor is used in the Monitor to measure Bench pressure and
ambient pressure when the pump is turned off. An integrated temperature
compensated pressure sensor MP1 is used. Its output voltage level is directly
proportional to the absolute pressure. To convert its output voltage into
0..2.5VDC ADC input range, precise divider R48 is used.
FiO. Sensor.
The FiO; sensor represents a millivolt range voltage source. U13B with
associated components is used to amplify the millivolt range sensor output
voltage into a 0..2.5VDC ADC input range. If the sensor is not plugged in, the
input of the operational amplifier U13B is at “ground” level, which indicates
“no sensor” condition. If the sensor is plugged in, the offset voltage from the
resistor divider R87-R88 shifts the input voltage high, indicating “sensor”
condition. The offset value is acquired and stored during the factory calibration
process.
Battery Voltage Level.
The battery voltage comes to the analog section through resistor R50 and is
buffered by U12D. Then it is directly digitized by the ADC. This information is
used by the microprocessor to generate an early low battery warning.
3-8 Capnocheck Plus Service Manual