Electrocardiography (ECG)/ Background
The heart is one of the vital organs in the human body. The heart behaves like a pump that circulates blood and oxygen around the body in order for the human body to function flawlessly. Waste products are removed from the body …show more content…
Differential amplifiers are used to amplify only the potential difference across two points (electrodes) and to make sure that noise from the input is not amplified.
Firstly we designed a circuit solely using operational amplifiers because this way we had better chance to find and amplify the desired signal for the first time. We chose TL071 amplifiers because of the high gain and low noise (CMRR) characteristics. This circuit configuration is also called instrumentation amplifier.
The first stage for the instrumentation amplifier is the input stage or buffer. The second stage is the differential gain amplifier. The input stage gives no common mode gain (no common mode noise) but high input impedance with a gain of G=1 + 2R1/Rg and it also buffers the gain stage.
The gain stage has low impedance and gives differential gain using the first stage output as input. The overall gain is G=(1+2R1/Rg) x R4/R3 = 1334 (Note:R3 = R, R4 = R5, R1 = R2) C1 and C2 act as a LPF f= 1/(2piRC)=51.3Hz. Figure 3 shows the circuit design. A 10Hz square signal was used as input for simulation. (Figure 4 and …show more content…
The signal strength and the base line highly depend on the connection between the electrodes and the skin. First we used electrodes made for TENS machine with resistance about 100 ohms which worked fine in a relatively noiseless environment but it turned out to be unproductive during the lab, due to the high level of electromagnetic noises. Finally, we used proper ECG electrodes with resistance of around 10 ohms. These high quality electrodes functioned very well, but unfortunately after a given time period we had to dispose them as they became useless. Possibly, because of the gel inside dried out and didn’t conduct anymore. Signal shape also depends on the position where the electrodes placed. Figure 9 shows the position of the electrodes placed on the “patient” chest to get the best signal. After we tried out other points like left or right wrist or right leg for the ground electrode (black wire) we ended up at the chest as the best