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:
Lower ADC demands 12-16-bit
Higher ADC demands, requires 19-bit due to the DC offset
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
*(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
I appreciate your reply, actually my application consists of a live streaming of the acquired data, therefore we need to access the outputs so we can do some experiments with it.
Thank you again,
Thanks for your message.
Have you seen the "write to file" tab within the ADI software?
This provides you with ability to store data from the device - is there a reason this is not sufficient?
I am an engineering student and currently working on an ECG project using your ADAS1000 hardware and labview software. the problem is that I need to access the output digital data of each electrode and I really need the VI file to carry on my research.
Thank you for your help, looking forward to hearing from you.