5.3 Back Features
See Figure4 on page 19.
Stand-The standat 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 beaccessed if necessary by opening the bat-
tery coverwith a small Philips screwdriver (see Section 6.1).
Serial Number Label-Also behind the stand isthe serial number label that
uniquely identifies the instrument.
5.4 Internal Features
The significantcomponents inside the 1521 are described here.
Battery-The 1521 has an internalrechargeable battery pack that can be re-
chargedby the AC adapter without removing the battery. It can be used and re-
chargedmany times. If necessary, it can be easily removed and replaced (see
Section 6.1).
Micro-controller-The1521 uses a micro-controller to control all its functions.
The micro-controller manages the measurement process, retrievesmeasurement
data from the analog-to-digital converter(ADC), places measurements and
other informationon the display, senses button actions, reads battery status in-
formationfrom the power control circuit, and handles communications through
the serial port.
PowerControl Circuit-The power control circuit manages the electrical power
that drivesall the circuits. It handles switching between the two sources of
power(DC input and battery pack), regulates voltages, monitors the state of
batterycharge, and manages battery charging.
Analog-to-DigitalConverter and Measurement Circuit-The ADC takes an
analog signalproduced by the probe and converts it to a digital value that can
be read by themicro-controller. The ADC used in the 1521 was selected for its
excellentreso lution, linearity,and stability. The measurement circuit built
around the ADCwas carefully designed for accuracy and stability to match the
ADC. The measurement circuit allows complete rejection of probe wire resis-
tance effectsthat would otherwise seriously limit accuracy. Offsets from
sources such as thermoelectricEMF are also completely rejected. This is done
using a current reversaltechnique with the probe current alternating at a rate of
one cycleper second. Self-heating is minimized by using low sensing currents.
ForPRTs the current is only 0.5 mA. For thermistors the current is only 0.005
mA.
1521 Handheld Thermometer Readout
User’s Guide
16