High-Performance, Stand-Alone ADCs for a Variety of Embedded Systems Applications
2 ADC Converter Function Pack Design Guide
ADC Converter Function Pack Design Guide 3
High-Performance, Stand-Alone ADCs for a Variety of Embedded Systems Applications
SELECTING THE RIGHT ADC
Selecting the most suitable A/D converter (ADC) for your
application is based on more than just the precision or
bits. Different architectures are available, each exhibiting
advantages and disadvantages in various data-acquisition
systems. The required accuracy or precision of the system
puts you in a category based on the number of bits required.
It is important to always design your system to allow for more
bits than initially required: if an application calls for 10 bits of
accuracy, choose a 12-bit converter. The achievable accuracy
of a converter will always be less than the total number of bits
available.
Depending on the system requirements, your accuracy might
be better expressed in micro-volts, decibels or LSBs (least
significant bits). A FFT showing the frequency spectrum of a
device can be useful in determining the noise performance of
a given device. All Microchip stand-alone ADCs show typical
performance data for AC specifications, such as THD, SINAD
and SNR. The following table shows performance, in dB and
V/V, for 8- through 24-bit converters.
Typically, an amplier is required if the magnitude of the input
signal is signicantly lower than the full-scale input range of the
ADC. However, by selecting an ADC with a higher resolution, the
need for an amplier can be eliminated.
Successive Approximation Register (SAR) converters typically
range from 8 to 16 bits. Delta-sigma converters (∆Σ) can
achieve an accuracy of up to 24 bits and will be covered in
more detail in a subsequent design guide.
The figure below shows the different architectures vs. bits and
bandwidth.
Two Application Examples:
Seismic Recording
16 different devices are connected to a central processing
unit to monitor vibrations. Each device measures the signal
at an extremely fast rate, less than 100 µS. High accuracy is
not required due to the large signal size, but speed is of the
utmost importance. A high-speed SAR converter would be the
best selection for this application.
Voice-Band Recording
The human ear can detect signals from roughly 20 Hz up to
20 kHz. If the application is a telephone intercom system
where high-fidelity audio is not a concern, 60-70 dB of dynamic
range is sufficient. Based on these bandwidth and dynamic
range requirements, either a medium speed (50-200 ksps)
SAR or delta-sigma converter would work in this application.
Performance Table — 8- Through 24-bit Converters
Architecture vs. Bits and Bandwidth
#
of Bits 2^n
LSB
(FS = 1V)
Resolution
(%)
Resolution
(ppm)
Resolution
(dB)
8 256 3.91 mV 0.391 3910 48.16
10 1024 977 µV 0.0977 977 60.21
12 4096 244 µV 0.0244 244 72.25
14 16384 61 µV 0.0061 61 84.29
16 65536 15.3 µV 0.00153 15.3 96.33
18 262144 3.81 µV 0.000381 3.81 108.37
20 1048576 954 nV 9.54E-05 0.954 120.41
22 4194304 238 nV 2.38E-05 0.238 132.45
24 16777216 59.5 nV 5.95E-06 0.0595 144.49
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