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Keysight Technologies
Instrument Design Validation and
Recommended Calibration Policy
White Paper
Introduction
What are Keysight Technologies, Inc. policies regarding the design of the
recommended performance test published in our Service Manuals? Why should
users have confidence in the overall performance of the instrument, even for
functions and ranges that don't seem to be included in the calibration procedure?
Although the following article addresses these questions from the perspective
of the manufacturing division responsible for digital multimeters, the general
principles also apply to other product-types.
Digital Multimeter Adjustment and Verification
Procedures
From time to time, customers or calibration laboratories may inquire about why
specific adjustment and verification points or procedures are selected for a
given instrument model. This discussion is intended to provide general back-
ground information with respect to the methods and philosophies that Keysight
Technologies utilizes when specifying these aspects of individual product
service procedures.
Calibration and verification procedures documented in the Keysight Service
Guide are created and reviewed by design, service, and quality engineers and
incorporate our detailed, proprietary knowledge of the DMM's internal hardware
and software design and sources of measurement error. Development of
procedures and selection of verification test points is based upon our extensive
statistical analysis of both characterization data gathered during design verifica-
tion testing and through on-going monitoring of production processes. During
Keysight manufacturing, significantly more verification data are gathered and
used to monitor product performance and to assure our outgoing product qual-
ity. The documented user procedures completely describe all steps required to
fully adjust an instrument to conform to its published accuracy specifications.
Philosophically, Keysight verification procedures are designed to achieve > 99%
confidence that the instrument conforms to all published measurement specifi-
cations and that it is fully functional for use. This high level of user verification
confidence is achieved by a multi-tiered approach as described below.
First, all accuracy verification procedures are preceded by checking basic opera-
tional readiness through executing the instruments internal Self-Test procedure.
This checks internal circuit paths for functional operation and is intended to
assure, with > 90% confidence, that the instrument has not experienced a
hardware failure and "should be expected" to meet all published measurement
specifications -- if the specified adjustment procedures have been followed
previously. Some non-measurement, user accessible, functionality (e.g. display,
keyboard, computer interface, etc.) cannot be completely verified by Self-Test
and are generally not addressed by measurement verification procedures.
Certain instrument models utilize internal, auto-calibration procedures that
should be executed before any performance verification checks are performed.
auto-calibration, when employed, automatically compensates for numerous
measurement gains and offset drifts due to operating temperature variation and
component aging effects. Auto-calibration utilizes internal transfer measure-
ments, relative to the instruments primary voltage and resistance reference
standards, to eliminate these measurement errors. Second, all zero offset cali-
bration points are verified including both front and rear input terminals, where
present, since separate offset values are stored for each during adjustment.
3
The third tier of performance verification confidence comes from verifying the
linear gain terms of each unique measurement path. For example, while two-
wire ohms and 4-wire ohms appear to the user as two independent measuring
functions, they in fact share near 100% of the same measuring circuits, differing
only in the offset portion of the measurement. The ohms current source, respon-
sible for the linear gain term of the measurement is shared in both functions.
Generically, gain verification is performed near the full range points using the
nearest commonly available value. For example, ohms full range values are in
multiples of 1.2 (e.g. 120 , 1.2 k, 12 k, etc.) while Keysight specified adjust-
ment and verification values are chosen in standard multiples of 1.0 (e.g.
100 , 1 k, 10 k, etc.) for ease of user support. The verification test points
and methods specified by Keysight are selected to achieve maximum perfor-
mance verification confidence while not requiring undue support or cost of
ownership burden on our customers.
The fourth element of the verification procedures is aimed at validating the
performance of other circuit paths not specifically addressed by the linear offset
and gain terms previously discussed. For example, this includes verification
checks of the analog-to-digital converter (ADC) linearity (guaranteed by design
and not adjusted) and of the ac signal conditioning path frequency response
which may be either wholly guaranteed by design or may be adjusted at a
single cross-over frequency. Since the same ADC is employed for all measuring
functions and ranges, its characteristic is verified in a single configuration
where the signal conditioning circuits have the least effect on the overall
measurement result. The ADC integral linearity characteristic can be verified
using several measurements across the complete scale (i.e. positive full scale to
negative full scale). Similarly, the frequency response of the ac section can be
verified at the accuracy band edges on a subset of the measuring ranges based
upon specific knowledge of the instrument's circuit topologies. In addition,
some ac measuring characteristics are determined by fixed, digital signal pro-
cessing algorithms (DSP) and therefore do not require user verification. These
behaviors have been verified earlier through extensive product design validation
testing.
In summary, modern instruments such as DMM's employ closed-box electronic
calibration methods to store and digitally process measurement correction
constants for linear error terms. High quality instrument designs minimize
non-linear error terms by design such that no user corrections are necessary
to compensate for these non-ideal behaviors. In addition, many traditionally
analog behaviors of instruments have been replaced by digital circuits, software
algorithms and digital signal processing techniques whose characteristics do
not change with time, temperature, etc. Therefore, many of the historical beliefs
and experiences of users and calibration laboratories that developed with past
generations of measuring instruments are becoming increasingly obsolete and
outdated; particularly when inferring sources of measurement error in modern
instrument designs. Since independent verification of every possible measured
value is, and always will be, impractical by end users, one must rely on the
guidance and integrity of the instrument manufacturer to specify appropriate
adjustment and verification procedures for the instrument given their detailed
knowledge of design limitations and instrument failure modes. As always, users
may augment the manufacturer's verification procedures, as they deem neces-
sary, to achieve higher verification confidence at application-critical measure-
ment points.
4
05 | Keysight | Instrument Design Validation and Recommended Calibration Policy - White Paper
myKeysight For more information on Keysight
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services, please contact your local Keysight
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Other AP Countries (65) 6375 8100
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United Kingdom 0800 0260637
For other unlisted countries:
www.keysight.com/find/contactus
(BP-05-19-14)
This information is subject to change without notice.
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