P
AGE
15
1054417, V
ER
. 01
Alice
®
PDx
TM
Service & Technical Reference Manual
if a DC shift occurs due to amplifier offsets or a small DC offset occurring in the signal input. The action of the
integrator causes the appearance of a single pole of highpass filtering with a cutoff frequency of 0.318 Hz to
appear in the response of the ECG signal conditioning circuit.
EEG/EOG
The EEG signal conditioners serve to amplify and filter the low-level “brain-wave” signals picked up by
electrodes placed on the scalp of the patient such that they are suitable for application to the microcontroller
ADC. There are a total of 5 EEG amplifiers in the Alice PDx system. They are:
•
EEG N1 R1
•
EEG N2 R1
•
EEG N3 R2
•
EEG N4 R2
•
EEG R1 R2
All EEG signal conditioners are designed to the following parameters:
•
Input voltage range: 1000
µ
Vp-p
•
Input coupling type: AC
•
Input impedance: 2 Megohm single-ended, 4 Megohm differential
•
Bandwidth: 0.08 Hz to 35 Hz (-3 dB)
•
Output voltage range: 0 to 3 VDC
•
Quiescent output voltage: 1.5 VDC
•
Total gain: 3000
Since all five signal conditioners function identically, with the exception of their input connections, the operation
of only one amplifier will be described here.
Signal from the electrodes enters the Alice PDx at J7-1 and J7-2 and pass through EMC/ESD protection
components CR19B, R209, C30, CR18A, C32, R210, C129, C130 and C132. The signal then passes through
a highpass RC filter consisting of C158, R63, C159 and R64. This filter serves to remove any DC offset due to
electrogalvanic action of the electrodes and has a cutoff frequency of 0.08 Hz. The signal is amplified by
instrumentation amplifier U1 with a gain of 150.
The amplified signal then passes to a four-pole active filter composed of amplifiers U2A and U2B. This filter
has a gain of 20, a cutoff frequency of 35 Hz, and is designed to minimize phase distortion of the signal.
The filtered signal next travels to amplifier U2D, where it is buffered prior to being sent to the analog input of
the system microcontroller.
A sample of the filtered signal is sent to an integrator formed by U2C. The output of the integrator feeds the
reference pin of U1 and causes the output of U2B to have a quiescent voltage equal to ANALOG_REF, thus,
removing any DC offset due to amplifier offsets.
NOTE
Refer to Alice PDx Analog Board Schematic sheets 4, 5, and 6,
located in the Schematics section beginning on page 53.