As far as I know there are two methods for coupling an Analog Front End for ECG to the patient electrodes. Can you tell me what are the main advantages of a DC coupled ECG system over an AC coupled system?
An AC coupled system has the 'advantage' of having lower ADC demands in that the DC offset of the ECG front end is mostly removed. Sometimes this 'offset' can be quite large, on the order several hundres of millivolts or more with the small ECG signal of 0.1 to 3 mV riding on top of this 'offset'. The offset itself does not remain level and often combines with niose and other environemental inputs such as EMI (electromagnetic intereference) and RFI(radio frequency interference.
An AC coupled system has sigificant issues however. To implement the traditional AC coupled systetm the front end must incorporate resistors and capacitors to achieve this function.Capacitors can become quite large at 0.05 hertz. IN addition, the ability to provide 'fast restore', where the baseline is quickly restored after an inpulse that saturates the ampifier can be an issue as well. An example would be the need to quickly see the QRS complex after a defibrillation pulse has been delivered. Present 'fast restore techniques' work well but usually require additional circuitry.
With a DC coupled system the Dynamic Range input requirements are more significant in that the number of useful bits needed is on the order of 19 or 20 versus 12-16 for an AC coupled system. The advantages of a DC coupled system are significant however.
The low end AC response can be done in the digital domain with minimal phase distortion and therefore can be moved 'up' from 0.05 hertz to a higher frequency which helps in many clinical respects. (ACC allows this dominant pole in specific domains to be moved to around 0.67 hertz if the phase response is linear). The recovery time from an input overload is now a function of amplifier recovery and ADC dynamic range and not at the mercy of the front end R/C component time constants, etc. Since DC architectures can now achieve the desired low noise peak to peak noise performance over a diagnostic bandwidth with low power and a high degree of integration, DC architectures are becoming the preferred method of diagnostic ECG front end designs. Usage of FIR filters and DSP techniques in general to remove residual baseline wander, denoising algorithms, etc. afford the designer far more degrees of freedom versus the AC coupled system.
Please let me know if you wish additional information
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