P
AGE
19
1054417, V
ER
. 01
Alice
®
PDx
TM
Service & Technical Reference Manual
B
ODY
P
OSITION
S
ENSOR
(A
CCELEROMETER
)
A three-axis accelerometer mounted on the digital board provides body position sensing by means of
determining the sensor’s attitude with respect to gravity. The accelerometer is oriented such that:
•
X is in the vertical axis
•
Y is in the horizontal axis
•
Z is in the front-to-back axis
16-
BIT
A
NALOG
TO
D
IGITAL
C
ONVERTER
A standalone 16 bit ADC (U30) was included to provide the high resolution needed for sensing respiratory
signals. The LTC1867L provides for up to 8 input channels, only four of which are used. The ADC
communicates with the system microcontroller via an SPI port.
The ADC’s internal reference is disabled and the device fed with an external 3.0 VDC reference.
A
NALOG
R
EFERENCE
S
UPPLY
Refer to Alice PDx Digital Board Schematic sheet 1.
The analog reference supply provides a voltage that is equal to VREF * ½ to all of the analog circuitry on the
digital and analog boards. This ensures that the quiescent outputs of all of the signal conditioners throughout
the system are held at one half the input range of the system ADCs.
P
OWER
S
UPPLIES
Battery voltage is converted to the various operating voltages needed by the Alice PDx by means of three DC
to DC converter circuits. These circuits supply operating voltage to the +3.3V_DIG, +3.3V_ANA and +5V
busses.
Battery voltage can vary from zero volts, for a fully discharged battery, to about 4.8 VDC, for a fully charged
battery. This poses special problems for the +3.3V supplies, as the input voltage can be less than, equal to, or,
greater than the output voltage, requiring different power supply topologies as battery voltage changes. The IC
chosen for the 3.3 VDC supplies is the LTC3440, by Linear Technologies. This part automatically reconfigures
itself as either a buck or boost converter, as needed to maintain voltage regulation.
NOTE
Refer to Alice PDx Digital Board Schematic sheet 1 located in the
Schematics section beginning on page 53.
NOTE
Refer to Alice PDx Digital Board Schematic sheet 1 located in the
Schematics section beginning on page 53.
NOTE
Refer to Alice PDx Digital Board Schematic sheet 2 located in the
Schematics section beginning on page 53.
