Novametrix Medical Systems 515 Service Manual page 25

Service Manual: Model 515 Theory of Operation:Analog Board #2
the frequency range of 50 - 120 Hz (line voltage noise). The other chai
(CH 2) detects DC noise which is caused by high ambient light levels,
channel also checks for non compatible probes (non SuperBright™).
The input for both noise detection circuits is taken from IC21 pin 8.
noise detection is performed by IC12 pins 1, 2, 3, C13, C15, R12, R16,
R17. The low frequency breakpoint is created by C15 and R16, the high
quency breakpoint is handled by C13 and R12. Resistor R17 is connecter
VREF-, the combination of R12 and R17 amplify VREF- by -0.5, this sets
output of pin 1 at the middle of the A/D converter's range (2.05 VDC).
The DC noise is detected by IC2 pins 5, 6, 7, C95, R13, R18, R2, and
The gain is set to -0.5 by R13 and R18, C95 filters out high frequency nois(
acquire a smooth DC output at pin 7. Resistor R19 is connected to the VR'
line to set the output to the middle of the A/D converter's range. If a non
perBright™ probe is installed the output of IC21 pin 8 will pulse negai
(photodiode reversed), this will forward bias D1 which in turn changes
gain to -100 through R2. The output will therefore saturate signalling the f
cessor that a non SuperBright™ probe has been installed.