Specifications
IQmark Digital ECG Operation Manual
50
The following table lists the positive and negative inputs for each channel.
Channel Number
Positive Input
Negative Input
CH0
LA
RA
CH1
LL
RA
CH2
V6
Wilson Center
CH3
V5
Wilson Center
CH4
V4
Wilson Center
CH5
V3
Wilson Center
CH6
V2
Wilson Center
CH7
V1
Wilson Center
The Wilson Center is a reference point combined with the three limb leads.
The purpose of the instrumentation amplifier is to provide a high Common Mode
Rejection Ration (CMRR), rejecting common mode signals such as 60 Hz line noise.
Filters and Gain Amplifiers
After the instrumentation amplifiers, the 8-channel ECG signals are passed into two
stages of filters. The first stage consists of eight first-order high-pass filters with a cutoff
frequency of 0.048Hz. The second stage consists of eight low-pass filters with a cutoff
frequency of 159Hz. The ECG signals are also amplified to match the AD converter input
dynamic range of 0-2.5V.
There are eight test points provided on the PCB board. The test points are labeled as
CH0, CH1, CH2, CH3, CH4, CH5, CH6 and CH7. The signals measured on these test points
are amplified with a gain of 125 and DC offset to 1.25Volt.
Analog to Digital Conversion
The AD converter used is ADC12138 from National Semiconductor. The AD converter is
12-bit plus sign serial I/O A/D converter with MUX and Sample/Hold. The amplified and
filtered 8-channel ECG signals are passed to the 8-channel inputs of the AD converter.
The sampling rate for each channel is 500Hz. The digitized ECG data are sent to the
microcontroller through the serial interface.
Microcontroller
The microcontroller is a PIC16LC67 chip from Microchip Technology. This chip has 8Kx14
OTP, 33 I/O lines and built-in serial interfaces for A/D converter and RS232. The
microcontroller receives the digitized ECG data from the AD converter and encodes the
data with packets and sends out to the PC through serial interface circuitry. The setting
for the RS232 is 8 bit data, 1 bit stop, no parity, and 115,000-baud rate and is usually
handled automatically. Settings for USB are automatic and are not user selectable.
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