ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 53

3.3.2 Functional Blocks of a Digital ECG Acquisition
System

The generalized block diagram of a digital ECG acquisition hardware module is
shown in Fig. 3.1 . It can be divided into three functional blocks connected in
sequence, viz., analog front end, AD conversion, and a processor with a bus
interface capability. The function of the analog front end is to reject (or at least
minimize) the artifacts associated with the biosignal at the input, amplify the
biosignal to the input span of the AD converter, and maintain galvanic isolation of
the patient from power line or any accidental transient voltage surge.
The first stage of the analog front end is patient isolation circuit, which prevents
any current from the acquisition circuit to flow through the patient body. The first
stage provides low offset, high CMRR instrumentation amplifier (INA). This is the
most important of the circuit since it determines the quality of the acquired signal.
The next stage is a low pass filter and a notch filter to block high frequency and
power line interference. If the AD converter is operated in unipolar mode of
operation, then a level shifter is included at the final stage of analog conditioning.
The output from this stage is 0-5 V. Today's ECG acquisition cards employ a
12- or 16-bit AD converter, which supplies the quantized samples to the high-end
microcontroller. The microcontroller provides some intelligent features to the
acquisition module like self-test, auto-calibration. However, its main function is to
deliver the data samples to the bus interface unit for delivery to the final storage
device, which may be a computer, personal digital assistant, mobile phone, iPad,
etc. For this, the microcontroller is provided with a small buffer memory and
associated peripherals. A graphical front end at the acquisition device may provide
some flexibility in selecting the sampling rate, duration of acquisition, etc.

Lead
system

Isolation Circuit

High end
Microcontroller
with bus
interface

Computer,
mobile
phone, PDA
etc for
acquisition

AD
Converter /
Quantizer

Instrumentation
Amplifier

Filter
Section

ECG acquisition module

Fig. 3.1 Block diagram of a digital ECG acquisition module