ETTC'2003 - SEE

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THE NEXT GENERATION AIRBORNE DATA ACQUISITION SYSTEMS
A MINIMUM SPECIFICATION SET FOR A/D CONVERTERS
Paul Sweeney Ph.D, ACRA CONTROL
KEY WORDS: FTI, ADC, ENOB, THD, SNR, DNL
ABSTRACT: The advent of a new generation of analog to digital converters (ADC’s)
provides the aerospace signal-conditioning engineer with many design advantages, trade-offs
and challenges for their next generation of signal conditioning systems. These advantages
include increased range, resolution, accuracy, channel-count and sampling rate. However, in
order to capitalize on these advantages, it is important to understand the trade-offs involved
and to specify these systems correctly.
Trade-offs include:
• Analog vs. Digital signal conditioning
• Implementation issues such as 12-bits vs. 16-bits (or even 24-bits)
• Topology issues such as multiplexers vs. multiple ADC’s
• Filter-type selection
• Sigma-Delta vs. Successive Approximation ADC’s.
Specification challenges include:
• Total DC error vs. gain and offset (and drift, excitation, DNL, crosstalk, etc.)
• ENOB vs. SINAD (or THD, SNR or Noise)
• Coherency issues such as filter phase distortion vs. delay
This paper will discuss some of these aspects and attempts to produce a succinct
specification for the next generation of airborne signal conditioning, while also outlining some
of the lessons learned in developing the same.
1. INTRODUCTION
Today, it is no longer sufficient to focus on DC accuracy as a benchmark for the quality of an
ADC module. The following may help to illustrate this reasoning:
At first glance it may appear reasonable to specify a 6th order Butterworth filter, such that
there is less than 60dB aliasing at the Nyquist rate for a sampling frequency of 1Hz. However
because this popular filter has phase distortion, some components are delayed by 2.5
seconds and some components are delayed by 3 seconds.
On an old 10-bit system, a cross-talk specification of 60dB might be acceptable. However, on
a 16-bit system this amounts to 64 counts of noise. Therefore, with this amount of crosstalk
in the system, using any more than 10-bits of resolution is a waste of time.
Older FTI systems (from the 1980’s) have extra circuitry for zero or null insertion or for
multiplexing known voltages into the system ADC’s. These are sold as features, but really
only serve to calibrate the instrument rather than the sensor. Today, this extra circuitry would
add more errors then it would remove.
Finally, this paper argues that a little rigor in the specification of both DC and AC
characteristics considerably reduces confusion and the opportunity for specmanship.