Fan Jiang - June 22, 2015
Electrocardiography(ECG) has a long history. Early ECG studies can be dated back to the 1800s, with a string galvanometer as the detector. In the following two centuries, ECG itself has undergone many improvements, and finally made its way into the mass medication market. In this report, we will cover some details on ECG and its application, as well as an attempt to obtain ECG using consumer grade electronics.
ECG in Short
The human electrophysiology system is complex-with billions of neurons emitting electric pulses simultaneously, picking up the signal of a specific neuron is nearly impossible. Luckily, our heart is strong enough that it has to be driven by a strong signal enough to be detected by a simple lab amplifier.
Figure 1 Typical ECG
Normal human ECG has 12 leads each representing the electric vector of heart activity projected on a specific axis. All those leads combined together makes up for an panorama of heart status.
The most commonly used lead is the Lead 1 lead, which is the electric potential difference between one's left limb and right limb. The signal in this lead is usually composed of the following entities: A P wave standing for atrial depolarization, a QRS complex for ventricular depolarization and a T wave for ventricular repolarization. Among all these entities, the QRS wave complex is the most observable characteristic, and is commonly used in heart rate analysis.
ECG in Practice
A modern ECG system usually consists of the following essential function blocks: a signal transducer (electrode), a signal preprocessing unit (or analog frontend, usually an isolated instrumentation amplifier), a signal processing unit (typically digital), and some external peripherals such as a display and a keyboard.
Designing a field-use ECG system is an art requiring extensive experience in both analog and digital fields of the designer. In the following section, we will discuss in detail about the design gold standards in practice.
ECG and Its Future
An undeniable fact is that, medical instruments including ECG will undergo a revolution in recent times. Stable, powerful while affordable digital signal processors has already made its way into the electrophysiology market. As more advanced ECG analysis algorithms finds their way to the market, it is highly likely that future cardiovascular diagnosis will be full automatic. At the same time, developments in the semiconductor industry makes smaller and cheaper analog ICs available, which allows for even more portable devices to be developed.
The analog frontend of an ECG system is basically a lab instrumentation amplifier consists of a low noise amplifier (LNA), a 50/60Hz notch filter and an analog buffer.
The LNA amplifies the µV-range ECG signals into a mV-range signal with more power. Here bandwidth is not a problem but proper isolation should be applied in order to reduce RF interference.
The notch filter kills the 50Hz city power grid interference. Generally a twin-T type filter is used but not recommended because of its low stability. For all types of filters, a small-value variable resistor/capacitor should be put in parallel with all value-sensitive parts. You also have to tweak the values carefully. If possible, use a digital filter to further deal with the 50Hz noise.
The analog buffer improved the driving capability of the filtered signal. This part is simply an op-amp.
The power supply system is usually a voltage regulator and a single supply to dual supply converter.
The voltage regulator should better be a linear regulator instead of a buck-boost regulator in the sake of lower noise. Typically this part is a 78xx series integrated regulator.
For the single-to-dual supply converter, two identical resistors is used to generate the virtual ground. The resistor-based voltage divider is buffered by op-amps to make sure the voltage ratio is identical in various load levels.
Battery power is recommended because of its lower inherent noise. Proper decoupling measures should always be taken into consideration.
The electrode system consists of many electrodes and the corresponding cables and jacks. For serious applications, conducting paste should be applied to the conducting area between the electrode and skin. All cables should be isolated with proper grounding.
According to the considerations above, we built a demo ECG system. Here is the performance of it.
This is a Limb I lead waveform with some 50Hz noise. The QRS complex can be readily observed. The signal is somewhat distorted especially for the T wave. This is probably caused by the twin-T filter for its bad phase property in margin frequencies.
Figure 2 Waveform Obtained with the Demo System
As there is no variable capacitors in our demo board, we cannot tweak the filter's center frequency. The measured center frequency is around 51.4Hz, but this value can only be used as a reference as the measurement isn't precise. The gain of the filter is around 2.0 in its pass band.
The gain of the LNA is pretty precise, which is exactly 101 to three significant figures. By disconnecting the notch filter, direct measurement of the LNA output shows extraordinary amount of 50Hz noise, which completely masked the desired signal.
Personally I do not recommend to use a simply analog filter as a notch filter. In practice, the city grid 50Hz signal can be in the range of 48~52Hz, according to grid load. Thus to design more stable notch filters, phase noise and pass band width has to be sacrificed. As stated above, a digital filter is recommended.
Isolation and Low Noise Design
When dealing with µV grade signals, low noise design is a must. Capacitor-based DC isolator should be put in all input lines. To reduce radio interference, the area of the cable loop should be as small as possible. If possible, preamps should be near the electrode, and the ground of all preamps should be the same, and the grounding resistance must be very low.
Debugging of Hand-soldered SMD Circuits
Solder everything at ~340˚C. If any op-amps saturate, double solder the feedback circuit.