User manual

The previous figure can be used to understand how SSH is used in the DBK17 SSH Card. The process is
as follows:
• Input signals pass through an instrumentation amplifier and into a sample-and-hold stage.
• When the sample enable line goes high, each channel’s sample-and-hold stage will “freeze”
the current analog value. The values for all channels are separately “latched” within 50 ns of
each other.
• The signals are held in a stable condition, while the multiplexer switches through all
channels.
• The multiplexer sends the signals [one-by-one] to be digitized by the analog-digital converter
(ADC) in the primary data acquisition device.
• The resulting data is a snapshot of conditions at an instant, but the multiplexing and analog-
to-digital conversion are spread over a longer time interval.
• The simultaneous sample and hold circuit allows you to gather up to 256 simultaneous
samples using sixty-four DBK17s.
Two-Point Calibration of a Temperature Measurement System
Note: In the following text a temperature measurement system consisting of a DaqBook, a DBK19
thermocouple card, and DaqView is referred to. However, it is important to note that the two-point
calibration method can be applied to measurement systems that consist of other acquisition devices
and software.
Overview
Two-point calibration makes use of correction constants
to eliminate offset and gain errors in a temperature
measurement system.
The graph shows an example for a temperature
measuring system, with temperature expressed in
degrees Celsius.
The sensor output is on the horizontal axis and system
response is on the vertical.
The graph illustrates the following bulleted points:
0
10
20
30
40
50
60
70
80
90
100
10 20 30 40 50 60 70 80 90 100
Temperature Sensor Output
(
C
)
°
Measurement System Out
p
ut (
C
)°
• The system has a linear response. Most thermocouple measurement devices have a software
driver, which corrects for the nonlinear output voltage of each thermocouple type. Normally,
there is no provision for linearity adjustment by the user.
• The system has an offset at 0°C, which can be eliminated by introducing an offset correction. This
is the first point of a two-point correction.
• The system has a gain error at 100°C, which can be eliminated by introducing a gain (slope)
correction. This is the second point of a two-point correction.
Suppose you have a temperature measurement system consisting of a DaqBook, a DBK19 thermocouple
card, and DaqView data acquisition software. The DBK19 was shipped with a diskette of calibration
constants. These calibration constants are specific to the DBK19.
1-12 Signal Management 886995 DBK Option Cards and Modules