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Mch ECG 3 in cc tham khoElectrocardiogram (ECG) Front End

Electrocardiogram (ECG) Front End - Successfully meeting the signal acquisition challenge

requires knowledge of the signal source, good design practice, and ICs with appropriatecharacteristics, features and performance.

Signal Acquisition Challenges

The action potential created by heart wall contraction spreads electrical currents from the

heart throughout the body. The spreading electrical currents create different potentials atdifferent points on the body, which can be sensed by electrodes on the skin surface using

biological transducers made of metals and salts. This electrical potential is an AC signal with

bandwidth of 0.05 Hz to 100 Hz, sometimes up to 1 kHz. It is generally around 1-mV peak-to-peak in the presence of much larger external high frequency noise plus 50-/60-Hz

interference normal-mode (mixed with the electrode signal) and common-mode voltages(common to all electrode signals).

The common-mode is comprised of two parts: (1) 50- or 60-Hz interference and (2) DC

electrode offset potential. Other noise or higher frequencies within the biophysicalbandwidth come from movement artifacts that change the skin-electrode interface, musclecontraction or electromyographic spikes, respiration (which may be rhythmic or sporadic),

electromagnetic interference (EMI), and noise from other electronic devices that couple intothe input. Some of the noise can be cancelled with a high-input-impedance instrumentation

amplifier (INA), like the INA326 or INA118, which removes the AC line noise common toboth inputs and amplifies the remaining unequal signals present on the inputs; higher IA

CMR will result in greater rejection. Because they originate at different points on the body,the left-arm and right-arm ECG signals are at different voltage levels and are amplified by

the IA. To further reject 50- and 60-Hz noise, an operational amplifier deriving commonmode voltage is used to invert the common-mode signal and drive it back into the patient

through the right leg using amplifier A2. Only a few microamps or less are required to

achieve significant CMR improvement and stay within the UL544 limit.

Three ECG electrodes connected to patient using CMOS devices with 5-V single supply

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Supply Voltage

As in most other applications, the system supply voltage in biophysical monitoring continuesthe trend toward low, single-supply levels. While bipolar supplies are still used, 5-V systems

are now common and trending to single 3.3-V supplies. This trend presents a significant

challenge for the designer faced with a 500-mV electrode potential and emphasizes theneed for a precision signal conditioning solution. While the following discussion concentrates

on the single supply design, the principles involved apply to bipolar designs as well. A list ofrecommended single and bipolar supply devices can be found below.

Frequency Response

Standard -3-dB frequency for patient monitoring is 0.05 Hz to 30 Hz, while diagnostic grade

monitoring requires 0.05 Hz to 100 Hz or more. All ECG front ends must be AC coupled toremove artifacts from the electrode offset potential, though important features of the ECG

waveform have extremely low frequency characteristics.

Electrode Potential

Because electrode potential can reach +/-500 mV, eliminating the effects of electrodepotential by AC coupling at low frequency allows for precise measurements. A DC restorator

amplifier in a feedback configuration nulls out the DC offset. If the left arm DC offset is+300 mV and the right arm electrode is 0-V DC, the differential input voltage is 300 mV.

Because the instrumentation amp has a gain of 10, 3 V appears at the output of the

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instrumentation amp. With a gain of 50 or more, the output amplifier would try to drive thesignal up to 150 V but never does because a feedback integrator applies an equal negative

voltage to the reference point. Using this linear summing effect, the 3-V positive offset iscancelled by the negative 3-V correction voltage. The result of this DC restorator is to turn

the original DC-coupled amplifier into an AC-coupled amplifier. Because the DC electrodeoffset has been removed, the output stage can amplify the signal to maximize the data

converter input range without becoming saturated.

Instrumentation Amplifier Requirements

Stability in low gain (G = 1 to 10)High common-mode rejection (CMR)

Low input bias current (IB)Good swing to the output rail

Very low offset and driftOperational Amplifier Requirements

Low noise in high gain (Gain = 10 to 1000)Rail-to-rail output

Very low offset and drift