Healthcare ICs
Volume 9, Issue 8 YOUR SEMICONDUCTOR SOLUTIONS RESOURCE

Optimize Design Criteria of Portable Ambulatory/Holter Monitors

Ambulatory-quality Holter monitors providing 1-lead ECG traces must be lightweight and small for patient comfort and privacy. Wireless Holter monitors equipped with a memory card can transfer data through a cell phone or LAN line-based base station. Data is read in a physician's office or clinic at predetermined times by uploading information without removing the monitor or the placement of electrodes. In addition to performance, reliability, low power consumption, and cost considerations, wireless ECG Holter monitors must be designed to support dedicated telemetry bands.

Solution
Both ac-coupled and dc-coupled design approaches have merit, and selection is often based on personal preference. However, the priority given to power, size, and precision will play a part in determining the design approach taken.

AC-Coupled Design Approach

An ac-coupled approach is typically used to maintain battery life and performance while decreasing the overall system size. The ac-coupling approach eliminates the need for a complex high-pass pole in software.

The AD8295 is an amplifier front end that combines signal acquisition, antialias filter, and right leg drive on a single chip. This highly integrated solution uses 50% less board space than competing solutions by incorporating a precision instrumentation amplifier with two uncommitted op amps and two precision-trimmed matched resistors in a single 4 mm × 4 mm chip scale package. The AD8295 delivers common-mode rejection of 80 dB to 8 kHz for all grades at G = 1. The on-board in-amp is gain programmable and uses a single resistor to set the gain from 1 to 1000, delivering 1 ppm/°C maximum gain drift (B grade), 8 nV/√Hz maximum input voltage noise at 1 kHz, and 0.25 µV p-p input noise (0.1 Hz to 10 Hz). A driven leg eliminates the need for a 60 Hz notch filter.

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AC-Coupled Design Approach

DC-Coupled Design Approach

If a design requirement is to retain information from dc to below 0.05 Hz (indicating there is a bad connection), a dc-coupled design approach is likely more appropriate than an ac-coupled approach. In addition, since an ac-coupling capacitor is not needed, the physical dimensions of the device can be smaller than an ac-coupled design.

The AD8235 is the industry's smallest, lowest power instrumentation amplifier. It has RRIO and can operate on voltages as low as 1.8 V. It is smaller than the tip of a pencil, yet offers 40 µA (max) supply current with 6 nA shutdown current. It is ideal for power efficient, portable, lightweight medical devices and consumer health products, including ECG monitors. The AD8235 offers low input bias current of 10 pA and high CMRR of 100 dB (G = 100). This performance is key for compact, wearable home-based medical devices that improve patient comfort while providing doctors with continuous and reliable patient monitoring. It is available in a 1.6 mm √ 2.0 mm standard bumped-die package in a reel format to simplify manufacture. Pricing: $1.59.

Complementary Components:

Complementary components for ac-coupled and dc-coupled ECG designs are available at www.analog.com/healthcare/ECG.

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