Respironics Alice PDx, Сервисное и техническое руководство

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1054417, V
ER
. 01 P
AGE
14
Alice
®
PDx
TM
Service & Technical Reference Manual
D
ETAILED
C
IRCUIT
D
ESCRIPTIONS
A
NALOG
PCA
ECG
The ECG subsystem consists of three signal conditioning circuits that receive biopotential signals from
electrodes on the patient’s body and convert them to signals suitable for application to the ADC in the
microcontroller. All three channels provide identical gain and frequency response and differ only in input
connections.
All ECG signal conditioners are designed to the following parameters:
•
Input voltage range: +/-4 mVDC
•
Input coupling type: DC
•
Input impedance: 10 Megohm single-ended, 20 Megohm differential
•
Bandwidth: 0.318Hz to 200Hz (-3 dB)
•
Output voltage range: 0 to 3VDC
•
Quiescent output voltage: 1.5VDC
•
Total gain: 375
The three ECG channels are ECG-I, ECG-II and ECG-V. Since each of the three signal conditioners contain
identical circuitry, only ECG-I will be discussed here.
In a normal configuration, electrodes are placed on the patient in the following locations: left arm, right arm, left
leg, right leg and chest. ECG-I measures the potential between the left arm and the right arm. ECG-II
measures the potential between the left leg (+) and the right arm (-). ECG-V measures the potential between
the chest electrode (+) and the average voltage of the left arm, right arm and left leg electrodes. Other ECG
channels (known as “leads”) can be derived by the algebraic addition and subtraction of the various channels
by software running in the microcontroller, or in the host computer.
Signal from the patient electrodes enter the Alice PDx unit at J16-1 (left arm) and J16-3 (right arm) and passes
through R1 and R21, which in conjunction with C7 and C22, form lowpass filters for EMC susceptibility
reduction. A Transient Voltage Suppressor (CR4) and conventional diodes CR1A and CR1B provide ESD
protection. Resistors R18 and R20 bias the left and right arm signals approximately 1.5 VDC to ensure these
signals are within the common mode range acceptable to U9.
Amplifier U9 is a low-drift instrumentation amplifier which amplifies the difference between the left and right arm
signals by a factor of 18.7.
The output of U9 is fed to a four pole lowpass active filter formed by U10A and U10B. This filter has a cutoff
frequency (Fc) of 200 Hz (-3dB) and a gain of 20. This filter was designed to have low phase distortion, to
prevent distortion of the ECG waveform. The filtered signal is buffered by amplifier U10D prior to being sent to
the microcontroller ADC.
Amplifier U10C forms an integrator which is used to remove any DC shift that may occur in the amplifier/filter
chain. The integrator has a Tc of 10 seconds and feeds back into the reference input of instrumentation
amplifier U9. The integrator causes the signal at the output of U10B to be centered about ANALOG_REF, even
NOTE
Refer to Alice PDx Analog Board Schematics sheets 1, 2, and 3,
located in the Schematics section beginning on page 53.