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 connections of a typical 5-lead (4 limb leads and 1 anterior chest lead) ECG measurement system that integrates respiratory and pacing detection functions. This configuration is typically used for portable telemetry ECG measurements or minimal lead setup for line-powered bedside instruments.

When measured at 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 μV-level measurement sensitivity. For systems, many medical standards require maximum noise to be no more than 30µV pp; however, designers often set this number lower. Therefore, all sources of noise must be considered when designing a solution that meets system-level requirements.

The ADAS1000 's noise performance is rated for a variety of different operating environments. The power supply must be designed so that it does not degrade overall performance. The ADP151 linear regulator was chosen due to its ultra-low noise performance (9 μV rms typical, 10 Hz to 100 kHz), which, combined with the power supply rejection performance of the ADAS1000, ensures that the noise generated by the ADP151 does not affect the overall noise performance.

The ADAS1000 five-electrode ECG analog front end (AFE) solves the challenges posed by 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-grade ECG measurements.

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

A single ADAS1000 supports 5 electrode inputs, making traditional 6-lead ECG measurements easy. Connecting a second ADAS1000 slave device in parallel adjusts the system to a true 12-lead measurement (consisting of 9 electrodes and 1 RLD). If multiple slave devices (3 or more) are added, the system can be The system adjusts to 15 lead measurements and even more.

breathe

The ADAS1000 integrates a digital-to-analog converter (DAC) for respiratory actuation at programmable frequencies from 46 kHz to 64 kHz, as well as an analog-to-digital converter (ADC) to simplify this complex measurement process. The measurement signal is demodulated and converted into amplitude and phase information, from which the corresponding respiratory information can be determined to obtain specific cable parameters. This circuit has a resolution of 200 mΩ using internal capacitors and higher resolution (<200 mΩ) using external capacitors. The circuit features a flexible switching scheme that allows measurement of one of three leads (I, II or III).

Pacing detection algorithm

The pace detection algorithm runs three instances of the digital algorithm on three of the four possible lead wires (I, II, III, or aVF). It operates on high-frequency ECG data in parallel with internal decimation and filtering. 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. ADAS1000 returns a flag to indicate whether the pacing signal is detected on one or more lead lines, and returns the height and width of the detected signal. When users wish 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 the filtered and decimated ECG data on the standard interface remains constant.

Low power consumption

ADAS1000 is designed for low power consumption and can measure 5 ECG electrodes with only 21 mW. To further reduce overall power consumption in applications such as battery-powered Holter and telemetry applications, all unused channels and features can be easily disabled to further reduce power consumption to 11 mW per ECG lead.

low noise

Low-noise performance is critical if correct diagnosis is required under varying conditions. End equipment needs to rely on the noise performance of ADAS1000 to comply with regulatory standards. ADAS1000 allows trade-offs between noise performance, power consumption and data rate, making it suitable for use in a variety of products. In line-powered ECG systems where power consumption is not a major issue, the performance of the ADAS1000 is also excellent.

Its noise performance can be optimized using the device's High Performance mode, which increases the sampling rate of the on-chip SAR ADC to 2 MSPS, resulting in a higher signal-to-noise ratio (SNR).

Flexible data rates

The standard serial interface can output all ECG related information, including lead off status, pacing, breathing and other auxiliary functions. A large number of 32-bit or 16-bit data words, collectively called a "packet" or "frame," is output on the serial SDO pin of the data bus. Different data frame rates are available (2 kHz, 16 kHz or 128 kHz) ensuring the ultimate simplicity of data acquisition tasks. The lowest data rate (2 kHz) enables more decimation capabilities, and the frame data rate is optimized for low noise performance. Data can also be read in skip mode, which reads a packet or frame from the device on the second or third word each time. Data rate is minimum 500 Hz.

The photo of the ADAS1000 evaluation board connected to the SDP board is shown in Figure 2.

The evaluation board is designed to provide 1-lead to 12-lead ECG measurements.

Batteries used in portable ECG applications

There are different types of batteries used in portable ECG devices, and in some cases AA or AAA batteries may be used to facilitate replacement or recharging.

Batteries add to the overall weight of the instrument. Since patient comfort is so important, reducing the size and weight of the overall solution and maintaining battery life are top considerations for portable ECG applications.

The latest products tend to use chemical batteries, such as lithium-ion batteries, and the battery life can range from a few hours to a few days, depending on the product.

The battery voltage range depends on the power supply range of the components in the system. ADAS1000 requires 3.3 V AVDD. Therefore, if the ADP151 voltage regulator is used, the battery must supply at least 3.7 V, which requires a margin of 400 mV. The nominal voltage of a lithium-ion or lithium-polymer battery is 3.7 V; however, the discharge voltage is approximately 3.2 V. Therefore, two stacks are required to ensure that the ADP151 reaches the minimum voltage of 3.7 V.

Choose the right power solution

ADAS1000 requires at least two power supply rails - AVDD and IOVDD. As shown in Table 1, the ADCVDD and DVDD rails are optional; power can be derived from the AVDD or IOVDD rails respectively using the ADAS1000's integrated on-chip LDO.

AVDD and IOVDD are powered from the 3.3 V supply on the evaluation board. Select 3.3 V for the IOVDD supply rail to maintain compatibility with the SPORT interface on the EVAL-SDP-CB1Z . If it is necessary to interface with a microcontroller operating at a lower supply voltage, the IOVDD supply voltage can be as low as 1.65 V.

Alternatively, if a more power efficient solution is required, the ADCVDD and DVDD internal supply rails can be disabled via a hardware pin (VREG_EN) on the ADAS1000 so that the ADCVDD and DVDD supply rails can be driven from an external supply. Since the ADCVDD supply rail powers the ADC on-chip, it must be kept as clean as possible and must not be used with noisy digital supplies.

Depending on the specific operating mode, the AVDD rail supply current powering a single ADAS1000 is typically between 8 mA and 15 mA, with all 5 channels enabled; inactive channels can be disabled to reduce power consumption.

A dedicated ADP151 operates both the AVDD and IOVDD supplies on the evaluation board. Note that each ADP151 can drive 200 mA and therefore can power other components in the system. The input to the ADP151 regulator comes from a 5 V rail that is used elsewhere on the board.

With appropriate filtering, a single ADP151 can provide power to both the AVDD and IOVDD rails, ensuring that the AVDD rail is not affected by any digital noise on the IOVDD rail.

The EVAL-ADAS1000SDZ evaluation board is designed to provide the required 5 V supply to the EVAL-SDP-CB1Z board at approximately 250 mA. The ADP2503 buck/boost DC-DC converter generates a 5 V supply rail from a 4.5 V to 5.5 V input supply connected to the board.

If this hardware is connected to the SDP board and powered by a battery, the total power consumption will quickly drain the battery.