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SECTION IV
TECHNICAL DESCRIPTION
(23005R02 instrument board / 48901 display)
4.00 GENERAL
The SCALE-TRONIX® model 4802 PEDIATRIC
SCALE utilizes the latest developments in electronic
scales and microcomputer technology to provide a
highly reliable and accurate weight scale designed to
be easy to use. This section of the manual describes
the technical aspects of the scale as an aid in
servicing.
4.10 LOAD CELL TRANSDUCER
The function of the load cell transducer is to convert
the weight applied to the weighing platform into an
electrical signal for further processing and
subsequent display by the scale. SCALE-TRONIX®
uses proprietarily designed load cells in most of its
scales to optimize performance and reliability.
The load cell employed in the model 4802 is known
as a “Double Ended Bending Beam (“DEBB”). It is
constructed as a double bending beam with two strain
gauges, forming a “half-bridge”, mounted on each
end of the beam, where the bending takes place in
response to weight applied. The ends are
interconnected to form the equivalent of a complete
“Wheatstone bridge” configuration. Additional
calibration and temperature compensating resistors
are added in the DEBB’s internal wiring.
4.20 SCALE ELECTRONICS
Scale electronics consist of the following:
1. Differential signal amplification.
2. Additional amplification and signal filtering.
3. Analog-to-Digital (A/D) converter and Clock
circuit.
4. Battery and support circuitry, voltage
regulators, power supplies, etc.
5. Microcomputer and support circuitry.
6. Display board with keyboard.
4.21 DIFFERENTIAL SIGNAL AMPLIFICATION
The weight dependent output signal produced by the
load cell transducer in response to weight applied to
the cradle is a “differential signal”, meaning it is the
voltage difference between the “+ Signal” and “-
Signal” leads. Integrated circuit U4, an
instrumentation amplifier, is used to interface to this
differential signal and amplify it.
The output signal from the load cell is applied to the
protection network consisting of diodes
CR4/CR5/CR6/CR7. These diodes prevent
destructive over voltages caused by static discharges
from damaging U4. A high frequency filter, formed by
L1/L2/C9/C10
couples the weight signal to the input of U4. In U4 the
differential signal is amplified by a factor of 100, and
converted to a “ground-reference” voltage for further
processing.
Capacitors C16/C17/C24 provide local bypassing of
the power supplies used by instrumentation amplifier
U4. Capacitor C18 furnishes compensation of U4 by
reducing amplification at higher frequencies.
4.22 ADDITIONAL AMPLIFICATION AND
SIGNAL FILTERING
Components for a second gain stage are included on
the printed circuit. Operational amplifier U5 is used to
provide additional gain and signal filtering. U5,
together with capacitors C14/C15 and resistors
R17/R18, form an active low-pass filter. This helps to
remove fluctuations in the weight signal caused by
movement of the patient on the scale. U5, like U4, is
“chopper-stabilized” to correct internal offset and drift
errors.
Some versions of the scale include the network of
potentiometer P2 and resistor R23 to provide a
variable “tare” adjustment to remove the weight of the
cradle. Resistor R22 provides a fixed amount of
“tare” adjustment to complement the P2 – R23
network. Tare offset is necessary on this scale
because the entire A/D range of +20000 counts is
needed for the scale’s 20000 gram (20,000 kilogram)
capacity.
Resistors R24/R25 are used to increase the gain of
the circuit. An additional low-pass filter stage is
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