5.3 Back Features
See Figure 4 on page 19.
Stand-The stand at the back of the 1521 can be flipped down to prop up the in
-
strument for better viewing.
Battery Compartment- Behind the stand is the compartment that contains the
battery pack. The battery pack can be accessed if necessary by opening the bat
-
tery cover with a small Philips screwdriver (see Section 6.1).
Serial Number Label-Also behind the stand is the serial number label that
uniquely identifies the instrument.
5.4 Internal Features
The significant components inside the 1521 are described here.
Battery-The 1521 has an internal rechargeable battery pack that can be re
-
charged by the AC adapter without removing the battery. It can be used and re-
charged many times. If necessary, it can be easily removed and replaced (see
Section 6.1).
Micro-controller-The 1521 uses a micro-controller to control all its functions.
The micro-controller manages the measurement process, retrieves measurement
data from the analog-to-digital converter (ADC), places measurements and
other information on the display, senses button actions, reads battery status in-
formation from the power control circuit, and handles communications through
the serial port.
Power Control Circuit-The power control circuit manages the electrical power
that drives all the circuits. It handles switching between the two sources of
power (DC input and battery pack), regulates voltages, monitors the state of
battery charge, and manages battery charging.
Analog-to-Digital Converter and Measurement Circuit-The ADC takes an
analog signal produced by the probe and converts it to a digital value that can
be read by the micro-controller. The ADC used in the 1521 was selected for its
excellent resolution, linearity, and stability. The measurement circuit built
around the ADC was carefully designed for accuracy and stability to match the
ADC. The measurement circuit allows complete rejection of probe wire resis
-
tance effects that would otherwise seriously limit accuracy. Offsets from
sources such as thermoelectric EMF are also completely rejected. This is done
using a current reversal technique with the probe current alternating at a rate of
one cycle per second. Self-heating is minimized by using low sensing currents.
For PRTs the current is only 0.5 mA. For thermistors the current is only 0.005
mA.
1521 Handheld Thermometer Readout
User’s Guide
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