Novametrix Medical Systems 515 Service Manual page 22

Theory of Operation;Analog Board #2288
Service Manual: Mode
The red and Ir DC signals also feed low pass filters, the red signal;
handles by IC18 pins 1, 2, 3 and the Ir is handled by IC18 pins 5, 6, 7. Thi
ters are configured for a gain of 1 at 1 Hz and dropping off the 0.7 at 10 Hz
breakpoints are controlled by C25, C27, R35, R37 for the red signal, and
C28, R36, R38 for the Ir signal. The red and Ir AC signals are developed b;
tecting the change in the DC signals, this is done through AC coupling har
by C23 (red) and C24 (Ir).
After the AC coupling the two signals are buffered. The red AC sign
buffered by IC15 (pins 8, 9, 10). The input to this buffer is switched to gn
by the FILTIN line going low. This closes the switch between pins 2,
IC25 which shorts out input resistor R33. This is done to short out the
pling capacitor C23 when the signal becomes too large due to a large chanj
the input signal (during an LED power change or when the patient m
etc.). The Ir signal is buffered by IC15 pins 12, 13, 14 and its input is sh(
to ground by the switch between pins 6 and 7 of IC25 which is also contn
by the FILTIN line (IC25 pin 8).
The two buffered AC signals are then multiplexed onto one line then ;
nately sampled, this is done instead of "matching" an amplifier for each s:
(which would be required for the critical red/Ir ratio calculations done tc
quire the SaOl value). The multiplexing is done by the SAl and SA2 1
This is handled by IC17 pins 9, 10, 11 for the red channel and IC17 pins 14
16 for the Ir channel. The ASl line (IC17 pin 9) is brought low for 2.5 n
this closes the switch between pins 10 and 11 enabling the red AC sign
feed IC16 pin 3 via coupling capacitor C98. The SA2 line (IC17 pin 16) (
ates in the same manner. When SA2 line goes low for 2.5 msec (SAl wi
high) the Ir AC signal feeds IC16 pin 3 via coupling capacitor C99. The
and SA2 lines operate alternately every 2.5 msec.
1.4.8 AC Gain and Offset Circuit
The red and Ir multiplexed signal from IC16 pin 1 feeds pin 3 of the
converter IC14. This is a dual 12 bit D/A converter which performs two di
ent functions. Channel A of IC14 which receives the multiplexed AC signt
configured as a gain DAC. Channel B of IC14 is configured as an offset 1
which controls offset voltages of the red and Ir multiplexed AC signal.
Channel A of the DAC (gain DAC) uses the reference voltage pin VR
(pin4, IC14) as the output amplifier's feedback line and the feedback res
pin RFBA (pin 3) as the signal input. This will give an input to output ga:
4095/n, where n is the 12 bit data programmed for that channel (a value (
4095). Channel B is configured as a standard D/A converter, its output wi
a set voltage corresponding to the data programmed for that channel.
VBREF pin (pin 12) is connected to VREF- which will give an output ol
+4.1 volts for the binary value 0 - 495. The output current from lOUTB (pir
is converted to a voltage at IC13 pin 7.