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ECG Electrocardiogram

Electrocardiogram (ECG or EKG) Solution from Texas Instruments

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ADC ADC Amplifier Amplifier Display Driver Signal Processor Comparator ∆Σ Converter Touch Screen Control Input Buffer Input Buffer Clock Isolation Wired: USB RS232 Logic Level Shift AC/DC Supply LCD Backlight Battery Management Low Noise Power DDR Power Core & I/O Power Green Mode Controller DSP/OMAP Wireless: Zigbee Bluetooth Ref MUX Temp Sensor

Design Considerations

TI's TMS320VC5505 evaluation module together with TI's electrocardiogram analog front-end module make up the new C5505 ECG Medical Development Kit (MDK) which provides developers access to a development tool set that offers a complete signal chain solution along with software to save months of development time and for portable patient monitoring applications that demand battery efficiency.

ECG System Functionality and Evolution

Basic functions of an ECG machine include ECG waveform display, either through LCD screen or printed paper media, and heart rhythm indication as well as simple user interface through buttons. More features, such as patient record storage through convenient media, wireless/wired transfer and 2D/3D display on large LCD screen with touch screen capabilities, are required in more and more ECG products. Multiple levels of diagnostic capabilities are also assisting doctors and people without specific ECG trainings to understand ECG patterns and their indication of a certain heart condition. After the ECG signal is captured and digitized, it will be sent for display and analysis, which involves further signal processing.

Signal Acquisition challenges:

  • Measurement of the ECG signal gets challenging due to the presence of the large DC offset and various interference signals. This potential can be up to 300mV for a typical electrode. The interference signals include the 50-/60-Hz interference from the power supplies, motion artifacts due to patient movement, radio frequency interference from electro-surgery equipments, defibrillation pulses, pace maker pulses, other monitoring equipment, etc.
  • Depending on the end equipment, different accuracies will be needed in an ECG:
    • Standard monitoring needs frequencies between 0.05-30 Hz
    • Diagnostic monitoring needs frequencies from 0.05-1000 Hz
  • Some of the 50Hz/60Hz common mode interference can be cancelled with a high-input-impedance instrumentation amplifier (INA), which removes the AC line noise common to both inputs. To further reject line power noise, the signal is inverted and driven back into the patient through the right leg by an amplifier. Only a few micro amps or less are required to achieve significant CMR improvement and stay within the UL544 limit. In addition, 50/60Hz digital notch filters are used to reduce this interference further.

Analog front end options:

  • Optimizing the power consumption and the PCB area of the analog front end is critical for portable ECG's. Due to technological advancements, there are now several front end options:
    1. Using a low resolution ADC (needs all filters)
    2. Using a high resolution ADC (needs fewer filters)
    3. Using a sigma-delta ADC (needs no filters, no amplifier aside from INA, no DC offset)
    4. Using a sequential Vs simultaneous sampling approach.
  • When a low resolution (16 bit) ADC is used, the signal needs to be gained up significantly (typically 100x - 200x) to achieve the necessary resolution. When a high resolution (24bit) sigma delta ADC is used, the signal needs a modest gain of 4 - 5x. Hence the second gain stage and the circuitry needed to eliminate the DC offset can be removed. This leads to an overall reduction in area and cost. Also the delta sigma approach preserves the entire frequency content of the signal and gives abundant flexibility for digital post processing.
  • With a sequential approach the individual channels creating the leads of an ECG are multiplexed to one ADC. This way there is a definite skew between adjacent channels. With the simultaneous sampling approach, a dedicated ADC is used for each channel and hence there is no skew introduced between channels.

Application Notes

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Selection and Solution Guides

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Tools and Software

Name Part # Company Software/Tool Type
Electrocardiogram (ECG) Analog Front End Module for the C5505 ECG Medical Development Kit TMDXMDKEK1258 Texas Instruments Daughter Cards
TMS320VC5505 DSP Evaluation Module TMDXEVM5505 Texas Instruments Development Boards/EVMs

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