The role of higher
integration of both channel count and analog function can be significant in
addressing the demands in analog signal acquisition for a variety of today's
medical applications. To effectively leverage the benefits of higher
integration and still achieve a desired cost/performance target requires an
understanding of both process advantages and design trade-offs most relevant to
the system of interest. This discussion presents a targeted comparison in the
architectural trends and design trade-offs as it relates to improved system
performance. We'll also cover lower cost-per-channel for analog data
acquisition systems of two very specific arenas in the medical field: ECG and
CT scanning.
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Electrocardiography
Electrocardiography (ECG) is a noninvasive method for
capturing and processing the electrical signature of the heart via skin
electrodes. ECG applications are broad and varied in their scope, and
therefore, so are the requirements for the analog data acquisition system. The
ECG signal and its harmonics are low in bandwidth (150Hz). The challenges in
ECG signal acquisition revolve primarily around external noise rejection on the
analog front-end (AFE), noise filtering, sampling, and signal processing by the
back-end conditioning circuitry, analog-to-digital converter (ADC) and
microcontroller unit (MCU).
Figure 1
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A typical low-end ECG data
acquisition system requires a minimum of two to three electrodes that are
sensed by a differential, analog front-end (AFE) gain block, a band pass filter
and analog gain block, and a lower-resolution (10-12 bit) successive
approximation register (SAR) converter. The strategy and degree to which noise
is removed from an ECG signal acquisition system is dependent on the overall
system cost target. The cost of non-clinical, lower-end ECG systems has been
dramatically reduced by analog integration. In fact, a disposable ECG patch
most likely will contain the AFE, filtering, gain amps and a low-resolution ADC
integrated into the MCU. Figure 1 shows a higher-level block diagram of a
typical ECG data acquisition system. Note the boxed area showing the analog
content that typically is fair game for integrating lower-end ECG data
acquisition systems.
Broader integration is also
the trend for medium to higher end, higher lead count ECG systems that require
lower noise AFEs and more bits of resolution in the data acquisition system, as
well as integrated ECG-specific functions such as right leg drive bias, Wilson
central reference (for chest lead measurements), lead off detection, and
provisions to separately process pace signals. One good example of this type of
integration is demonstrated by the ADS1298, a low-power, low-noise,
simultaneous sampling, 24-bit ADC that includes all of these ECG functions
along with a front-end multiplexor that allows the user to toggle between the
electrode inputs, input test signals, right leg drive reference, supply voltage
and an internal temperature sensor.
Figure 2. Click here for a larger version
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Computed Tomography Scans
Medical computed tomography (CT) is a method used to
process two dimensional X-rays of images ("slices") that result in a
three-dimensional representation of a target area of the body. Capturing a
"slice" of data is initiated when X-rays transmitted through the body are scintillated
and strike a dense photodiode array, resulting in a photocurrent that is
sensed, amplified, sampled and filtered by a data acquisition system comprised
of an integrating AFE, ADC, and a significant amount of post processing to take
the numerous slices of 2-D X-ray data and reconstruct the desired image.
Unlike ECG, CT scanning applications are largely clinical and are
subject to a very specific set of cost/performance trade-offs. A better image
can be achieved with better signal-to-noise ratio (SNR). Better SNR comes by
increasing the amount of signal more than the noise. More signal can be
obtained by increasing the number of photodiode detectors. Therefore, three of
the key design constraints in CT scanning signal acquisition are SNR integral
nonlinearity (INL) and channel density (i.e., area in mm2 / # channels). Since
surface densities of state-of-the art photodiode sensor arrays in modern CT
scanners have dipped below the 1mm2/channel threshold, a similar reduction in
the per channel surface density of the data acquisition
system translates to an increase in the amount of image data (i.e., signal)
captured per slice. As a result, this increase in density often means that the
photodiode and an "integrating" AFE (see Figure 3) can be located within a
closer proximity to each other, which also means that the parasitic capacitance
(Cp) of the connection from the photodiode to the AFE can be further minimized.
Since the voltage noise of an integrating front-end photodiode is a function of
the ratio between the total capacitance seen at the inverting node of the
integrator (C
in), minimizing this capacitance improves the overall SNR of the
data acquisition system.
One competing effect of the
increased channel density in the integrated data acquisition system of a CT
scanner is the drift of the offset and AFE transfer function integral
nonlinearity due to internal self-heating. The CT data slices represent
snapshots in time that rely on the absolute accuracy of the data acquisition
system based on an initial system calibration to reconstruct a clean 3-D image
of the desired object. For this reason, its internal drift must be kept at a
minimum through low-power design techniques, packaging and component layout.
Likewise, because of the "optimized" proximity of the data acquisition system
to the photodiode array, any self-heating effects may also induce self-heating
in the photodiode, which can dramatically impact the responsivity of the
photodiode and the overall SNR of the data acquisition system.
Figure 3
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While the comparison of ECG and CT scanning spans a wide range of
functional complexity and design challenges, both are driven to reduce cost
without sacrificing performance in next-generation designs. The role of modern
process technology and functional integration can be significant in achieving
this goal, whether it is done by integrating the AFE + ADC on the chip or
increased channel count. For the engineer to truly realize the cost/performance
benefits of advances in integration requires a solid understanding of the
engineering design challenges most important to his or her design.
Matthew William Hann is Precision Analog Applications
manager at Texas Instruments.
For more information:
www.ti.com/medical-ca
View system block diagrams:
β’
ECG system www.ti.com/ecgsbd-caβ’
CT scanner www.ti.com/ctsbd-ca
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