Scale-Tronix 5002 - Technical Description of Scale Components; Readout Electronics Overview; Differential Signal Amplification; Additional Amplification and Signal Filtering

Readout 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.
7. Printer assembly.
8. Printer interface board.
9. Printer controller board,
4.21 DIFFERENTIAL SIGNAL AMPLIFICATION
The weight dependent output signal produced by the
load cell transducers in the weighing frame 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 cells is applied to the
protection network cansisting of diodes
CRA4/CRS/CR6/CR7. These diodes prevent
destructive overvoltages caused by static discharges
from damaging U4, A high frequency filter, formed by
L1/i2/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-referenced"
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 an optional second gain stage may
be added on the printed circuit board. If included
operational amplifier US is used to provide additional
gain and signal filtering. U5, together with capacitors.
C14/C15 and resistors R17/R18, forms an active low-
pass filter. This helps to remove fluctuations in the
weight signal caused by movement of the patient on
the scale. US, like U4, is “chopper-stabilized" to
correct internal offset and drift errors.
Resistors R24/R25 (optional) are used to increase
gain; resistors R26/R27 (optional) are used for gain
reduction. An additional low-pass filter stage is
furnished by resistor R34 and capacitor C23.
4.23 ANALOG-TO-DIGITAL (A/D) CONVERSION
Integrated circuit U6 is the analog-to-digital converter.
Included on this integrated circuit are auto-zero
functions, auto-polarity, and the digital and analog
functions necessary to perform dual slope integration
conversion to 20,000 counts (4% digits}. The weight
signal voltage is applied to the analog input (pin 10) of
Us.
À reference voltage for the conversion is applied to pin
2 of U6. The reference voltage, nominally 1 Volt, is
derived from the load cell transducer excitation
voltage, by the divider network consisting of resistors
R29, R30, and potentiometer P1. Adjusting P1 sets
the "span" or weight calibration of the scale.
The system clock, applied at pin 22 of U6, is used to
다
pease:
Past
2
pass
κώνο
|
Son
il
SERA,
INTEGRATE
yanit
v
5
ve
TORRE
|
sao
|
veto
mm
nom
mises
ARARLAR
ARADAN
AID CONVERTER TIMING DIAGRAM
precisely time and control the phases of the dual slope
conversion process. Refer to the converter timing
diagram when reading the following description.
18 5002, 5102, 5602, 6002, 6102, 6202, 6502, 6702