A wireless Holter monitor—small enough to fit on the back side of an electrode—provides more accurate signals than traditional designs that typically require the recording monitor be carried in a pocket or in a pouch worn around the neck or waist because noise and interference are greatly reduced The circuit is inexpensive and is capable of providing diagnostic-quality, 1-lead ECG traces A driven leg eliminates the need for a 60 Hz notch filter Patient comfort and privacy are greatly improved because all circuitry can be worn under clothing.
Data is encrypted and uploaded automatically every few minutes to a collection and analysis system on-site for hospital, nursing facility, or intermediate care facility in-patients Patients at home can go to their physician’s office or clinic at predetermined times (perhaps daily or weekly) to upload information without removing the monitor or the placement of electrodes Wireless Holter monitors also can be equipped with a memory card for data transfer through a cell phone or LAN line-based base station in the home In addition to performance, reliability, low power consumption, and cost considerations, wireless ECG Holter monitors must be designed to support dedicated telemetry bands so that the ECG data from the monitor transfers quickly, accurately, and securely to the data collector for evaluation.
The Wireless Medical Telemetry Service (WMTS) band in North America, and other unlicensed ISM bands, provide a dedicated spectrum to ensure an interference-free, reliable link for data transfer Analog Devices’ ADF7021 high performance, narrow-band ISM transceiver IC supports the WMTS band and the 433 MHz, 868 MHz, and 915 MHz ISM bands It achieves best-in-class receiver sensitivity of –123 dBm at 1 kbps and includes on-chip T/R switch, VCO tank, RF/IF filters, and fully automatic AFC and AGC To achieve multiyear battery life, the ADF7021 can be put into ultralow power sleep mode, drawing typically less than 0.1 µA of current.
ICs for Low Power, Low Cost Holter Monitors Support Standard Wireless Bands
When used in combination with a low power microcontroller, average standby currents of less than 2 µA can be achieved. A WMTS optimized reference radio design (EVAL-ADF7021DBZ6) includes schematics and layout files, and can be a starting point for your wireless ECG Holter monitor design.
The AD623 is an integrated single-supply instrumentation amplifier that delivers rail-to-rail output swing and low power consumption (1.5 mW at 3 V). The center node is used for accessing the residual common-mode signal. The AD8500 micropower precision CMOS operational amplifier features a maximum supply current of 1 µA, a maximum offset voltage of 1 mV, and a typical input bias current of 1 pA. It operates rail-to-rail on both the input and output. The device can operate from a single-supply voltage of 1.8 V to 5.5 V or from a dual-supply voltage of ±0.9 V to ±2.75 V. The AD8641 low power, rail-to-rail output JFET amplifier offers high input impedance, precision performance, and low cost. The input impedance needs to be much larger than the 681 kΩ resistor attached to its input. The AD7466 is a 12-bit ADC that features extremely low power consumption in a small package. Power dissipation is 480 µW @ 3.6 V/50 kSPS. It is available in a 6-lead SOT-23 package. For designs requiring wider dynamic range and a higher signalto-noise ratio, the AD7685 PulSAR® ADC offers 16-bit resolution and power dissipation of 1.35 mW @ 2.5 V/100 kSPS. It is available in a 3 mm × 3 mm QFN (LFCSP).
These options (shown in the figure below) require a low power microcontroller. Another option for this portion of the signal chain is to replace these two ICs with the ADuC7022. The ADuC7022 incorporates a 16-bit/32-bit microcontroller that has a high performance 1 MSPS multichannel ADC integrated on the same chip. The ADC provides the necessary dynamic range to meet medical standards and to detect very small ECG waveform changes. The ARM7® architecture has 32-bit registers enabling it to perform real-time FIR filters on the ECG signal.