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How High Performance Modular
Instruments Enable Greater
Applications and Smaller Form Factors
By Michael Cliord
Following generations of custom benchtop instrumentation, the transition
continues toward modular instruments with more flexibility, software
control, and smaller form factors. However, lower power consumption in
tandem with meeting noise and measurement accuracy targets remains
a challenge.
One element of the change away from benchtop instruments is the ability
to enable mobility. The large custom and rack-based systems have
limitations from a logistical point of view. The breakup of cabinet/desktop
equipment into smaller nodes enables custom configuration and optimizes
the instruments for the environment and geography of the test site or subject.
Improving the mobility of the measurement instruments by breaking
them into smaller nodes reduces wiring and installation headaches.
Wiring connections to local mobile instruments is easier than tracing
long cables back to a central rack or bench instrument. Time spent
verifying wiring and fixing miswired connections is recovered. Despite
the changing form factors, the need for optimum test performance,
certainty, and accuracy remain.
Application Areas
A modular platform instrument with high dynamic range is the 21
st
century
equivalent of the measuring tape. The instrument delivers the measurement
capability required to further innovation, research, and development
across a broad spectrum of specialties.
X
Testing within research and development fields of material science,
from the structural analysis of the blades of a windmill to the health,
well being, and electrical output of it’s turbine.
X
Measuring the outputs of strain/piezoelectric transducers, condition-
ing those voltages and enabling quantitative analysis for structural
health and materials development, and providing clear measurement
without interference.
X
Measurements for automotive cabin noise. Digitizing the output of
microphones placed in-cabin during the prototype development, right
through to enabling faster, more accurate control loops that increase
production throughput on the factory floor.
X
Electrical testing:
• Audio measurement enabling the development of advanced
microphone modules and speakers for voice-activated control
and operation.
• Electrical test within ATE for both passive and active electronics
where parametric measurement accuracy and speed relate to the
cost of the test.
X
EEG needs extremely high dynamic range over a specific bandwidth
close to dc. Lower power is required to pack many hundreds of
simultaneous measurement channels to be packed into a small
form factor.
These wide ranging applications have equally wide ranging channel
counts. Standard 8-channel modules in industrial applications extend to
512 channels and beyond for an EEG measurement. Scaling the front-end
measurement design to a large number of channels, while maintaining
simultaneous sampling, is key. It is the basis for the data that guides a
generation of research, development, production, and end operation.
Creating smaller form factor housings, whilst holding measurement
channel density requires power efficiency. Increasing the dynamic range
of the analog-to-digital converter
(
ADC
)
and the chain, which precedes
it to 110 dB, while keeping current consumption in check is a constant
battle. Balancing dynamic range, input bandwidth, and current consumption
isn’t easy.
A new ADC subsystem supported by the capabilities of the AD7768 and
AD7768-4 has emerged. It provides the capability to digitize to wider
bandwidths with higher accuracy than before and to do so with fidelity
and synchronized sampling across multiple channels. It also provides
tools to ease thermal challenges and strike the right balance of dynamic
range, input bandwidth, and current consumption in high dynamic range
modular system design.
Recongurable Thermal Footprint, Software
Programmable Measurement Bandwidth
The AD7768 can adapt to the measurement situation. Heat, decreasing
air space, and the absence of active cooling are all constraints of modular
instruments, which the AD7768 eases using built-in operating modes for
fast, median, and eco power scaling. For a given input bandwidth, the
user may decide to expend more or less power, reducing heat within the
module. An example would be digitizing over an input bandwidth of
51.2 kHz. Such a bandwidth is popular for FFT-based analysis as it
provides an integer bin size within the FFT output. The AD7768 features
a brick wall digital filter frames that required input bandwidth. A low ripple
pass band and a steep transition band combine with full attenuation at
frequencies just beyond 51.2 kHz, meaning there is no fold back from
around the Nyquist frequency. For the AD7768, the user can choose to
operate in either fast or median mode. The decision is between current
consumption and dynamic range depending on which is most constrained
for the system. Let’s take a look:
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