Traditionally ECG applications have tended to be AC coupled systems, more recently DC coupled approaches are becoming more popular for various reasons. There are advantages and disadvantages to each approach which this FAQ will explain.
The main motivating factor for an AC coupled system is to remove the large DC component of the signal prior to the ADC. The offset itself depends on the type of electrodes used. For a typical stainless steel type electrode, this offset could be as much as +/-1V, while for AgAgCl electrodes, the offset will be within +/-300mV.
In a DC coupled system, the signal of interest rides directly on the DC electrode offset. While it does require a higher performance ADC, a DC coupled approach offers signification advantages; it offers greater flexibility, ensures fast restore of ECG to some event such as de-fibrillation pulse or surgical knife.
The ADAS1000 ECG Front end device is designed for applications requiring a single supply, DC coupled approach. The ADC used within the ADAS1000 is a oversampled SAR (19-bit at 2kHz data rate).
A quick summary is shown below, taking into account some key elements:
|
Parameter |
AC Coupled |
DC Coupled |
|
ADC resolution |
Lower ADC demands 12-16-bit |
Higher ADC demands, requires 19-bit due to the DC offset |
|
External components |
Resistors/capacitors at the input, the capacitors become quite large for 0.05Hz HPF. |
HPF can be easily done in digital domain |
|
Fast restore * |
Hardware fast restore techniques needed. E.g. circuit may switch out the large input capacitor |
No issue, can restore the ECG signal very quickly, well within the required time frame |
| Front end gain | High-gain can be used prior to digitization | Only low-gain can be applied prior to digitization |
*(to defib shock or electro-cautery, ablation). The instrument will need to restore the baseline quickly after the impulse so the physician can quickly see the return to QRS complex or otherwise