Key Features of ECG Amplifiers

Jul 30, 2021

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A differential amplifier is employed as ECG amplifier instead of the typical electronic amplifiers. The reason why the electronic amplifiers aren’t employed in electrocardiograph (ECG) is because they create an interference problem when they are used to measure small bioelectric potentials.

A differential amplifier can be considered as two amplifiers having separate inputs but they have a common output terminal, thus the differential amplifier delivers the sum of the two amplifiers output voltage as illustrated in the figure below:

Figure 1(a) Differential Amplifier

The differential amplifier amounts to two amplifiers with separate inputs and a common output. Both amplifiers have the same voltage gain but their output voltages have 180° phase difference. This means that their resulting common voltage gain is zero when they have the same input source. The output of one amplifier cancels the output of the second amplifier due to 180° phase difference. However, if the two amplifiers do not have the same gain, then this complete cancellation is impossible, thereby a small residual common output voltage remains. In case the input voltage of one of the amplifiers is grounded, and input voltage is applied to the other amplifier, then the input voltage will appear as output voltage with the gain of the amplifier. The gain is called the differential gain of the differential amplifier. The ratio of the differential gain to the common mode gain is called common mode rejection ratio of the differential amplifier.

In order to measure bioelectric signals that occur as potential difference between two electrodes a differential amplifier is employed as shown in the figure 1(b). The bioelectric signals are applied between the non-inverting and inverting inputs of the differential amplifier.

Figure 1(b) the circuit of differential amplifier for the amplification of bioelectric signals

The input signals are amplified to the differential gain of the amplifier. For the noise signals, both the inputs appear as though they are connected to a common input source. Hence, the noise signal is amplified by much smaller gain i.e. almost rejected.

The key features of the amplifier are:

Three input-terminals, out of which one is made as the reference potential by grounding.
It can measure the voltage difference between two points, which are varying in amplitude at different rates and in different patterns.
It can reject common mode interference signals which are invariably picked up by the electrodes from the body along with the useful bioelectrical signals.
It has high input impedance so as to obtain a high common mode rejection ratio.
It has good stability and versatility. High stability is achieved as it is insensitive to temperature changes. It is versatile as it can be used in many applications.
It has low power consumption.
Its voltage gain depends upon the difference of input signals.
The electrode resistance should low.