P
AGE
17
1054417, V
ER
. 01
Alice
®
PDx
TM
Service & Technical Reference Manual
The DC voltage obtained across C105 is fed to amplifiers U21A and U21B via capacitor C117. C117 removes
any DC offset that may be present in the belt signal while U21 amplifies the signal to a level suitable for
application to the ADC.
It should be noted that the effort belt signals are fed to the LTC1867L 16 bit ADC, rather than the internal ADC
of the microcontroller. This was done because it was felt that respiratory signal would need a greater dynamic
range than other signals.
P
RESSURE
/S
NORE
/R
ESPIRATION
S
ENSOR
The pressure sensor in Alice PDx allows the measurement of pressure at the patient’s mask due to a CPAP or
ventilator, and the detection of both snore and respiration via the use of a nasal cannula.
When a pressure tap is connected from the Alice PDx to the patients’ mask, pressure over the range of -5 to 40
CM can be read from the sensor into the microcontroller via the sensor’s I2C port.
When a cannula is connected, patient snore and respiration signals are detected by using a direct connection
from the sensor’s bridge output pins, which is amplified in instrument amplifier U18. Here, the differential
voltage is amplified by a gain of 82.
The amplified signal is then fed to two sets of amplifier/filter circuits for deriving the respiration and snore
signals.
The respiration signal is detected by taking the U18 output and passing it first through a single pole RC filter
consisting of C81 and R17. This filter has a cutoff frequency of.03 Hz. The signal is then amplified by a factor of
3.15 by U17A. The respiration signal passes through a 15 Hz, 2 pole active filter, consisting of U17B and
associated components. The amplified and filtered signal is then sent to the LTC1867L 16 bit ADC (U30) on
the digital board.
The output of amplifier U18 also feeds the snore detection circuit. The amplified signal. Passes through U23A,
where it is highpass filtered at 100 Hz and amplified with a voltage gain of 50. The filtered signal next passes
through U23B, which is configured as a gain of 1 inverting amplifier. This amplifier configuration was chosen to
provide a simple way to increase the gain of the signal path, if needed, simply by changing the values of
resistors R27 and R12. U23C and U23D provide additional voltage gain (8) and lowpass filtering (180 Hz) of
the signal, prior to application to the analog to digital converter U30, located on the digital board.
The pressure sensor used in the Alice PDx offers extremely stable performance over a wide range of
temperatures. Stability of the sensor is enhanced by an onboard temperature sensor and DSP that correct the
sensor’s output as needed, to limit error to +/-2% over the temperature range of 0 – 60 °C.
Since the digital output of the sensor is only updated at a rate of <100 Hz and is limited to 12 bits, it was
necessary to gain access directly to the bridge outputs and apply external signal processing to obtain the snore
and respiration signals.
NOTE
Refer to Alice PDx Analog Board Schematic sheet 11located in the
Schematics section beginning on page 53.
