6.0 DRAWINGS
Figure 6-1. 502A Preamp Block Diagram
Figure 6-2. 502A Postamp Block Diagram
APPENDIX A - TRANSMITTER
ACCURACY SPECIFICATIONS
The complex current-transmitter circuitry necessary to amplify,
isolate, protect, and offset weak input signals while consuming
only small amounts of power can distort the signal in many ways.
Additional accuracy limitations occur in thermocouple transmitters,
which require precise cold-junction compensation and large Zero-suppression
ranges in order to obtain good sensitivity and linearity for high
temperatures.
Many transmitter data sheets omit key accuracy factors and/or
express performance in percentage values without mentioning the
full-scale value. Design limitations can be disguised by such
"specsmanship"; the 502A specifications, however, are detailed
in order to present the complete performance accuracy.
For a given thermocouple type, input errors are logically expressed
in degrees (rather than microvolts), and output errors are readily
expressed in microamperes, since output is current. Transmitter
users are rarely interested in microamperes. Therefore, these
output current errors are translated back to input degrees as
a percentage (or ppm) of the selected Span.
Another fundamental division of errors is that of independence
or dependence on Zero and Reading. Resistor aging and tempco mismatch
in the Zero and Voltage Reference circuits, for example, will
produce errors which increase with Zero suppression but which
are independent of the amount of Reading (value above the Zero).
Resistor aging and tempco mismatch in the amplifier gain (feedback)
circuits, however, will usually affect both Zero and Reading accuracy,
while amplifier gain tempco variations are important to just the
Reading stability. A complete error specification, therefore,
needs a term proportional to Zero (suppression) and a term proportional
to Reading.
For thermocouple transmitters, the Cold-Junction Compensation
(CJC) is never perfect, even when factory-tailored over wide ambient
excursions with curvilinear adjustments, as in the 502A. This
error component is readily stated as a percentage of the ambient
temperature excursion from the nominal temperature at which the
Zero was set (assuming, as in the 502A, that the Zero potentiometer
has ample resolution on all Zero and Span ranges). For transmitters
with restricted turndown ratios (low Zero Suppression capability),
the tempco errors may be lumped into a single error term.
In addition to these three components of tempco (ambient temperature
effects), there are other possible errors, often referred to as
"hysteresis," "repeatability," "drift," or "time" errors. No statistically
significant errors of these types have yet been observed for the
502A, which utilizes a solid-state, band-gap input voltage reference,
matched-pair input PNP transistors, integrated-circuit current
source and imbalance control, and matched-tempco bridge resistors.
The 502A also provides a variable-tempco output adjustment (factory-set)
which eliminates many of the errors lumped in this category for
other units. The 502A specification includes a 0.2°C tolerance
for the calibration accuracies.
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