Front Panel Display Pcb And Controls - Nellcor NPB-190 Service Manual

Technical Supplement
S8.7 AC Ranging
S9

FRONT PANEL DISPLAY PCB AND CONTROLS

S9.1 Display PCB
S9.2 Membrane Keypad
S-8
In order to achieve a specified level of oxygen saturation measurement and to
still use a standard type combined CPU and A/D converter, the DC offset is
subtracted from each signal. The DC offsets are subtracted by using an analog
switch to set the mean signal value to the mean of the range of the A/D converter
whenever necessary. The AC modulation is then superimposed upon that DC
level. This is also known as AC ranging.
Each AC signal is subsequently amplified such that its peak-to-peak values span
one-fifth of the range of the A/D converter. The amplified AC signals are then
filtered to remove the residual effects of the PWM modulations and, finally, are
input to the CPU. The combined AC and DC signals for both IR and red signals
are separately input to the A/D converter.
Visual patient data and monitor status is provided by the Front Panel Display
PCB. At power up, all indicators are illuminated to allow verification of their
proper operation.
There are two sets of three 7-segment displays. One set displays %SpO
other displays pulse rate. A decimal point immediately to the right of either
display indicates that an alarm limit for that parameter is no longer set at the
power on default value.
In between the two 7-segment displays is a 10-segment blip bar. The blip bar
illuminates with each pulse beat. The number of segments illuminated indicate
the relative signal strength of the pulse beat. A tone will accompany each pulse
beat. The sound of the tone will change pitch with the %SpO
measured.
Four LEDs and icons are also located on the Front Panel Display PCB. An LED
illuminated next to an icon indicates a function that is active. Functions indicated
by the LEDs are AC/Battery Charging, Low Battery, Alarm Silence active, and
Pulse Search.
A membrane keypad is mounted as part of the top case. A ribbon cable from the
keypad passes through the top case and connects to the UIF PCB. Six keys allow
the operator to access different functions of the NPB-190.
These keys allow the user to select and adjust the alarm limits, cycle power to
the unit, and to silence the alarm. Alarm volume and alarm silence duration can
also be adjusted via the keypad. A number of other functions can be accessed by
pressing the Upper and Lower Alarm Limit buttons simultaneously and then
selecting the desired option with the Adjust Up or Adjust Down button. These
functions are discussed in greater detail in Section 4.
and the
2
level being
2