Image Processing Reference
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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 common-
mode potential is higher in one of the two voltage sources. Therefore, part of the common-
mode 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,
2004):
Wished output due to the differential amplification on the ECG signal.
Common-mode signal not wished due to the CMRR is not infinite.
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,
INA131 and TLC1541, are less expensive.
Fig. 6. ECG Signal Amplifying Module Circuit
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