Electrocardiagram (ECG)

TI's new ADS1298 provides eight channels of PGA plus separate 24-bit delta-sigma ADCs, a Wilson center terminal, the augmented Goldberger terminals and their amplifiers, provide for a full, standard 12-lead ECG integrated analog front end. The ADS1298 reduces component count and power consumption by up to 95 percent as compared to discrete implementations, with a power efficiency of 1 mW/channel, while allowing customers to achieve the highest levels of diagnostic accuracy.

ECG System Functionality and Evolution

Basic functions of an ECG machine include ECG waveform display, either through LCD screen or printed paper media, and heart rhythm indication as well as simple user interface through buttons. More features, such as patient record storage through convenient media, wireless/wired transfer and 2D/3D display on large LCD screen with touch screen capabilities, are required in more and more ECG products. Multiple levels of diagnostic capabilities are also assisting doctors and people without specific ECG trainings to understand ECG patterns and their indication of a certain heart condition. After the ECG signal is captured and digitized, it will be sent for display and analysis, which involves further signal processing.

Signal Acquisition challenges:

• Measurement of the ECG signal gets challenging due to the presence of the large DC offset and various interference signals. This potential can be up to 300mV for a typical electrode. The interference signals include the 50-/60-Hz interference from the power supplies, motion artifacts due to patient movement, radio frequency interference from electro-surgery equipments, defibrillation pulses, pace maker pulses, other monitoring equipment, etc.
• Depending on the end equipment, different accuracies will be needed in an ECG:
• Standard monitoring needs frequencies between 0.05-30 Hz
• Diagnostic monitoring needs frequencies from 0.05-1000 Hz
• Some of the 50Hz/60Hz common mode interference can be cancelled with a high-input-impedance instrumentation amplifier (INA), which removes the AC line noise common to both inputs. To further reject line power noise, the signal is inverted and driven back into the patient through the right leg by an amplifier. Only a few micro amps or less are required to achieve significant CMR improvement and stay within the UL544 limit. In addition, 50/60Hz digital notch filters are used to reduce this interference further.

Analog front end options:

• Optimizing the power consumption and the PCB area of the analog front end is critical for portable ECG's. Due to technological advancements, there are now several front end options:
  • Using a low resolution ADC (needs all filters)
  • Using a high resolution ADC (needs fewer filters)
  • Using a sigma-delta ADC (needs no filters, no amplifier aside from INA, no DC offset)
  • Using a sequential Vs simultaneous sampling approach.
• When a low resolution (16 bit) ADC is used, the signal needs to be gained up significantly (typically 100x - 200x) to achieve the necessary resolution. When a high resolution (24bit) sigma delta ADC is used, the signal needs a modest gain of 4 - 5x. Hence the second gain stage and the circuitry needed to eliminate the DC offset can be removed. This leads to an overall reduction in area and cost. Also the delta sigma approach preserves the entire frequency content of the signal and gives abundant flexibility for digital post processing.
• With a sequential approach the individual channels creating the leads of an ECG are multiplexed to one ADC. This way there is a definite skew between adjacent channels. With the simultaneous sampling approach, a dedicated ADC is used for each channel and hence there is no skew introduced between channels.