Highly integrated ECG front end for patient monitoring systems (circuit diagram)

This circuit is a highly integrated electrocardiogram (ECG) front end for battery-powered patient monitoring applications. Figure 1 shows a top-level block diagram of the physical connection of a typical 5-lead (four limb leads and one pre-thoracic lead) ECG measurement system that integrates breathing and pulse detection. This configuration is typically used for portable telemetry ECG measurements or minimum lead setup for line powered bedside instruments.

When measured on the skin surface, the ECG signal amplitude is small, typically 1 mV. Important information about the patient's health and other parameters is contained in that small signal, so the device is required to have a measurement sensitivity of μV level. As far as the system is concerned, many medical standards require a maximum noise of no more than 30 μVp-p; however, designers often set this value lower. Therefore, all noise sources must be considered when designing a solution that meets system level requirements.

The rated noise performance of the ADAS1000 is available in a variety of different operating environments. The power supply must be designed to ensure that overall performance is not compromised. The ADP151 linear regulator was chosen for its ultra-low noise performance (9 μV rms typical, 10 Hz to 100 kHz). The power supply rejection of the ADAS1000 ensures that the noise generated by the ADP151 does not affect the overall noise performance.

Figure 1. Simplified functional block diagram of the ADAS1000 for a typical 4-electrode+RLD or 5-lead configuration (all connections and decoupling not shown)

Circuit description

The ADAS1000 five-electrode ECG analog front end (AFE) addresses the challenges of a new generation of low-power, low-noise, high-performance tethered and portable ECG systems.

The ADAS1000 is a highly integrated chip consisting of five electrode inputs and a dedicated right leg drive (RLD) output reference electrode designed for monitoring and diagnostic level ECG measurements.

In addition to supporting the basic components of monitoring ECG signals, the ADAS1000 is equipped with functions such as breath measurement (thoracic impedance measurement), pacing artifact detection, lead/electrode connection status, and internal calibration.

A single ADAS1000 supports 5 electrode inputs for easy traditional 6-lead ECG measurements. Parallel connection of the second ADAS1000 slave device allows the system to be adjusted to true 12-lead measurements (composed of 9 electrodes and 1 RLD). If multiple slave devices (3 or more) are added, The system adjusts to 15 lead measurements even more.

The Breathing ADAS1000 integrates a digital-to-analog converter (DAC) for respiratory drive at a programmable frequency of 46 kHz to 64 kHz, and an analog-to-digital converter (ADC) to simplify this complex measurement process. The measurement signal is demodulated and converted into amplitude and phase information, and the corresponding breathing information can be determined accordingly, thereby obtaining specific cable parameters. This circuit uses an internal capacitor with a resolution of 200 mΩ and a high resolution with an external capacitor (200 mΩ). The circuit has a flexible switching scheme that allows one of the three leads (I, II or III) to be measured.

Pacing Detection Algorithm The pacing detection algorithm runs three digital algorithm instances on three of the four possible lead wires (I, II, III, or aVF). It runs in parallel with internal decimation and filtering for high frequency ECG data. The algorithm is designed to detect and measure pacing artifacts with widths ranging from 100 μs to 2 ms and amplitudes from 400 μV to 1000 mV. The ADAS1000 returns a flag indicating whether the pacing signal was detected on one or more leads and returns the height and width of the detected signal. When users want to run their own digital pacing algorithms, the ADAS1000 provides a high-speed pacing interface that provides ECG data at extremely fast data rates (128 kHz) while filtering and extracting ECG data on the standard interface. constant.

The low-power ADAS1000 is designed for low-power applications and requires only 25 mW to measure five ECG electrodes. To further reduce overall power consumption in applications such as battery-powered Holter and telemetry, all unused channels and features can be easily disabled to further reduce the power consumption of a single ECG lead to 11 mW.