**3. Digital ECG**

Building a device to get and process the ECG signal must consider the signal characteristics. According to (Cuesta, 2001; Vidal & Pavesi, 2004), facing individually each part of the global problems is a technique applicable in order to get good practical results.

Figure 4 presents each part or block of a basic digital ECG according to reviewed literature

Fig. 4. Blocks Diagram of a Basic Digital ECG.

A DSP Practical Application: Working on ECG Signal 157

For these reasons, common mode rejection ratio (CMRR) rate is a desirable characteristic of an amplifier working on differential mode. On a day today practice, a problem denominated contact impedance disbalance appears (Townsend, 2001) that is produced when there are different interfaces impedances between the skin and electrodes in a form that the commonmode potential is higher in one of the two voltage sources. Therefore, part of the commonmode voltage is worked as differential voltage and amplified according to the amplifier gain. This occasionally produces saturation on the next amplifying module stage, if the amplification module were composed by more stages. This voltage, which is generally continuous, can be eliminated using a simple high-pass filter. Hence, the output voltage of the differential amplifier would consist of 3 components (Townsend, 2001; Vidal & Pavesi,

x Wished output due to the differential amplification on the ECG signal. x Common-mode signal not wished due to the CMRR is not infinite.

INA131 and TLC1541, are less expensive.

Fig. 6. ECG Signal Amplifying Module Circuit

x Common-mode signal not wished due to the disbalance on the impedance contact. (Wells & Crampton, 2006) indicate that weak signals require an amplification of 1000 at least to produce adequate signal levels for future works on it. (Vidal & Pavesi, 2004) used an instrument amplifier model INA131 which presents a fixed CMRR of 100, and according to the associated datasheet it is adequate for biomedical instrumentation. The analog to digital conversion stage (A/D conversion) is always done when the signal is amplified. The electronic schemes of a digital electrocardiographic device according to (Vidal & Gatica, 2010) are presented on figures 6 and 7, respectively. (Vidal & Pavesi, 2004; Vidal & Gatica, 2010) use the TLC1541 A/D converter. It is necessary to indicate that both electronic items,

2004):

(Cuesta, 2001; Vidal et al., 2008; Vidal & Gatica, 2010) where the most important part corresponds to the amplifying module because of a bioelectrical signal that represents a low potential, and sophisticated amplifiers are required for obtaining and recording it (Vidal & Pavesi, 2004; Vidal et al., 2008; Vidal & Gatica, 2010).

The following sections present experiences building a device for getting the ECG signal, and works related to processing ECG signal.
