Novametrix Medical Systems 515 Service Manual page 23

Service Manual: Model 515
Theory of Operation:Analog Board
The negative input of IC16 pin 2 is configured as a summing amplifier,
output from IC13 pin 7 is amplified with a gain of -.25 by R29 and R30,
VREF- line is summed and amplified by -0.125 by R30 and R31. By sumn
these together IC16 pin 1 can give an adjustable offset corresponding to
data programmed into DACB of IC14. The range of the offset is ± 512 mV.
The multiplexed red and Ir AC signal present at IC16 pin 3 is set to a ^
of 1 (IC16 pin 1) by R30 and R32. This signal is added to the offset volt
programmed by DACB of IC14, to allow software to adjust the DC level of
AC signal to the middle of the ADC input range. Capacitor C22 reduces
high frequency noise from appearing at the output.
The output from IC16 pin 1 is amplified to an optimum level by DACA
IC14 and will appear as an output voltage at IC13 pin 1. This output feeds i
plifier 1C12 pins 8, 9, 10, which is configured with a gain of 4 through R14
R21. The VREF- line also feeds this amplifier and is amplified with a gain i
2 created by R14 and R20, this results in a -t-2.05 volt DC level at pin 8
IC12. This sets the signal at the middle of the ADC range so the offset D
(DACB of IC14) can fine tune the signal to the exact center of the A
(IC14) input range.
When the system is running the red and Ir AC signals are alternat
switched into the gain DAC (DACA of IC14) every 2.5 msec. The gain val
programmed into DACA come from a preprogrammed table in software c
taining a number of different settings. The gain table is used by both the
and Ir signals but the red and Ir gains are not necessarily the same at any (
time. So before the software switches the red or Ir signal into the gain DA(
sets up the appropriate gain for corresponding signal in DACA.
For an optimum reading at the ADC input each gain setting for both s
nals has its own particular offset value. These offset values are in the form o
red and Ir offset table and are obtained during initial production testing of
front end board. The offset tables are stored in the E^PROM and are tra
ferred to RAM during the initialization of the monitor after it has bt
switched on. In the same way that the gain is set up before each signal
switched into the gain DAC, the corresponding offset is also set up.
1.4.9 ADC Circuit
The front end board monitors eight different parameters through the use
an 8 channel, 8 bit A/D converter (ICll). This converter runs on a 142.8 K
clock which is derived on the digital board. The eight channels are selected
addressing the A0-A2 lines (pins 17-19) with the appropriate address.