Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord
EEE512J2 – Electronic Product Design Page 33 Chapter 7: Electronic / Software Design
7.0. ELECTRONIC / SOFTWARE DESIGN
7.1. Design Considerations
Accuracy, dependability, and precision are an absolute must if the device were to be used for diagnostic, or
other medical purposes. Any small fluctuation in the waveform generated could carry critical diagnostic
value, thus it is extremely important that the clinician can confidently and fully rely on the equipment. This
means that the ECG must faithfully display the cardiac signal exactly as it exists in reality, such that any
irregularity detected did in fact arise from an unhealthy cardiac cycle, not from the equipment that was used.
Therefore, there were many special considerations that had to be taken into account when designing the
ECG Monitor.
Noise:
First and foremost in these considerations is the effect of noise. Noise interference in the signal detection
process would be detrimental to the experiment, as the ECG signal is at such small amplitudes it could easily
be masked by noise related fluctuations. Therefore in order to detect the signal accurately, there must be
strict limitations on the acceptable level of noise allowed, and every possible attempt must be made to
minimise this level and reduce the effects of noise on the data acquisition process.
Signal Amplitude:
Another consideration that strongly influenced the design of the ECG is the fact that the cardiac signal
generated has a very small peak amplitude. (As stated above, this is the very signal attribute that makes
noise control so vital). Considerable amplification is necessary if there is any use to be made from the
cardiac signal in terms of analysis and output. Also, the small size of the signal plays a very influential role in
the approach to creating a system of visual output. Caution has to be taken to effectively differentiate
between actual changes in the signal amplitude as opposed to a random variation in noise amplitude.
Low frequency:
Because the signal that is generated from the cardiac muscle has such a low bandwidth, it is very important
that the ECG have a good low frequency response. This is because any shifts in the frequency of the
detected signal, especially the S-T portion of the waveform, carry critical diagnostic value.
7.2. System Powering
Figure 7.2a shows the circuit diagram; clearly a 5V regulator is used to generate a 5 volt DC output to power
the circuit. The 0.1µF capacitors absorb line noise, while the 100µF capacitors are used for storage in the
event of a minor drop in power (milliseconds) the circuit operation will not be affected.
C2
0.1uF
C3
0.1uF
C4
100uF
50V
C1
100uF
50V
Vi Vo
Com
MC7805CT
8 to 20 VDC 5 VDC
Figure 7.2a. System powering circuit
This means that the circuit has now got a wide operating voltage range as 8 to 20 volts DC will power the
circuit. Note there are higher spec 7805 chips available that can operate up to 30 volts DC, if there is a need
for a higher voltage range.
The system can be powered from a battery source (e.g. PP3 9V), or a DC power supply (e.g. 12V). It has
been decided, not to design a complete power supply unit from scratch, but to use commercially available
units. It is important to remember that voltage regulators are not efficient and as the input voltage increases
the least efficient they become, energy is lost in the form of heat. A heat sink is normally required to keep the
chip within its maximum operational temperature.