CN0308

The ADAS1000 five-electrode ECG analog front end (AFE) addresses the challenges of next generation, 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 and is designed for both monitor and diagnostic quality ECG measurements.

In addition to supporting the essential elements of monitoring ECG signals, the ADAS1000 is equipped with functionality such as respiration measurement (thoracic impedance measurement), pace artifact detection, lead/electrode connection status, and internal calibration features.

A single ADAS1000 supports five electrode inputs, easily enabling a traditional 6-lead ECG measurement. Paralleling a second ADAS1000 slave device allows scaling of the system to a true 12-lead measurement (made up of nine electrodes and one RLD), while adding multiple more slave devices (three and above) scales the system to a 15-lead measurement and beyond.

Respiration

The ADAS1000 has an integrated digital-to-analog converter (DAC) for respiration drive at a programmable frequency of 46 kHz to 64 kHz, and an analog-to-digital converter (ADC) that simplifies this difficult measurement. The measurement is demodulated and converted to magnitude and phase from which respiration can be determined, given the specific cable parameters. The circuit has a resolution of 200 mΩ using the internal capacitor, and higher resolutions (<200 mΩ) using an external capacitor. The circuit has a flexible switching scheme allowing measurement on one of three leads (I, II, or III).

Pace Detection Algorithm

The pace detection algorithm runs three instances of a digital algorithm on three of four possible leads (I, II, III, or aVF). It runs on the high frequency ECG data in parallel with the internal decimation and filtering. It has been designed to detect and measure pacing artifacts of widths ranging from 100 μs to 2 ms and amplitudes of 400 μV to 1000 mV. The ADAS1000 returns a flag that indicates pace was detected on one or more of the leads, as well as the measured height and width of the detected signal. When users wish to run their own digital pace algorithm, the ADAS1000 has a high speed pace interface providing the ECG data at a fast data rate (128 kHz), with the filtered and decimated ECG data remaining on the standard interface.

Low Power

The ADAS1000 is designed for low power and requires only 21 mW to operate five ECG electrode measurements. To further minimize overall power dissipation in applications such as battery-operated Holter and telemetry, any unused channels or features can be easily disabled to further reduce power to as low as 11 mW for one ECG lead.

Low Noise

Low noise performance is critical for appropriate diagnosis of different conditions. The ADAS1000 noise performance is required to support end equipment regulatory standards. The ADAS1000 allows tradeoffs between noise performance, power, and data rate, making it suitable for a wide variety of products. The ADAS1000 performance also excels in line-powered ECG systems where power is not a major concern.

Device noise performance can be optimized using its high performance mode, where the sample rate of the on-board SAR ADCs is increased to 2 MSPS, thereby giving a higher signal-to-noise ratio.

Flexible Data Rates

The standard serial interface outputs all the information related to the ECG, including LEADS OFF status, pace, respiration, and other auxiliary functions. The large number of 32-bit or 16-bit data words, collectively known as a packet or frame, is output on the serial SDO pin of the data bus. Different data frame rates (2 kHz, 16 kHz, or 128 kHz) are available to ensure ultimate ease in data capture. The slowest data rate of 2 kHz allows for more decimation and is the optimum frame data rate for low noise performance. It is also possible to read data in the skip mode that reads the packet or frame from the device every second or third word. The slowest data rate is 500 Hz.

A photograph of the ADAS1000 evaluation board connected to the SDP board is shown in Figure 2.

The evaluation board has been designed to offer 1-lead to 12-lead ECG measurement.

Batteries Used in Portable ECG Applications

The types of batteries used in portable ECG equipment vary, and, in some cases, AA or AAA batteries are found, allowing for easy replacement or recharging.

Batteries contribute to the overall weight of the instrument. Because patient comfort is important, minimizing overall solution size and weight while maintaining battery life is a prime concern in portable ECG applications.

Newer products are likely to use battery chemistries, such as lithium ion, and achieve operating time from a few hours to a number of days, depending on the product.

The battery voltage range depends on the supply range of the components within the system. An AVDD of 3.3 V is required for the ADAS1000. Therefore, if the ADP151regulator is used, the battery must supply at least a 3.7 V minimum, which requires a 400 mV headroom. The nominal cell voltage of a lithium ion or lithium polymer battery is 3.7 V; however, the discharge voltage is about 3.2 V. Therefore, a stack of two is required to guarantee a 3.7 V minimum for the ADP151.

Selecting the Right Power Solution

The ADAS1000 requires a minimum of two power rails, AVDD and IOVDD. As shown in Table 1, the ADCVDD and DVDD rails are optional, and these supplies may be derived from the AVDD or IOVDD rails, respectively, using the on-chip LDOs within the ADAS1000.

On the evaluation board, AVDD and IOVDD are powered with 3.3 V. The 3.3 V was chosen for the IOVDD rail to maintain compatibility with the SPORT interface on the EVAL-SDP-CB1Z. If interfaced to a microcontroller operating at a lower supply voltage, the IOVDD supply voltage can be as low as 1.65 V.

Alternatively, where a more power efficient solution is required, the ADCVDD and DVDD internal rails can be disabled using the hardware pin (VREG_EN) on the ADAS1000, which allows the ADCVDD and DVDD rails to be driven by the external supplies. Because the ADCVDD rail powers the ADCs on the chip, it must be kept as clean as possible, and it must not be shared with noisy digital supplies.

Depending on operational mode, the supply current on the AVDD rail for a single ADAS1000 is typically between 8 mA and 15 mA with all five channels enabled; unused channels can be disabled to lower power.

A dedicated ADP151 is used for both the AVDD and IOVDD supplies on the evaluation board. Note that each ADP151 is capable of driving 200 mA and can therefore supply other components within the system. The input to the ADP151 regulators comes from a 5 V rail generated on the board for other purposes.

A single ADP151 can supply both the AVDD and IOVDD rails if appropriate filtering is added to ensure the AVDD rail is kept free of any digital noise on the IOVDD rail.

The EVAL-ADAS1000SDZ evaluation board has been designed to provide power to the EVAL-SDP-CB1Z board that requires 5 V at approximately 250 mA. The ADP2503 buck boost dc-to-dc converter generates the 5 V rail from an input supply to the board of 4.5 V to 5.5 V.

If this hardware is battery powered with the SDP board connected, the overall power consumption quickly discharges the batteries.