13
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 (4-1/2 digits). The
weight signal voltage is applied to the analog input
(pin 10) of U6.
A 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
precisely time and control the phases of the dual
slope conversion process. Refer to the converter
timing diagram when reading the following
description.
4.23.1 PHASE 1, AUTO ZERO
During auto zero, the errors in the analog
components (offset voltages of buffers, comparators,
etc.) will be automatically nulled out. This is
performed by internal logic that disconnects the input
pins (9 & 10) from the applied analog signal, connects
them to ground, then closes an internal feedback loop
such that offset error information is stored in the “auto
zero” capacitor, C21. Also during this phase,
“reference capacitor” C22 is charged to the voltage
present on “Vref” (pin 2 of U6).
4.23.2 PHASE 2, SIGNAL INTEGRATE
The input signal is reconnected and then integrated
for exactly 10,000 clock pulses. On completion of the
integration period, the voltage V is directly
proportional to the input voltage, corresponding to the
weight applied to the scale. Capacitor C20 is the
integration capacitor, with resistor R32 setting the
integration current. At the end of this phase the input
signal polarity is determined.
A/D CONVERTER TIMING DIAGRAM
4.23.3 PHASE 3. REF. INTEGRATE, SIGNAL
DEINTEGRATE
The input to the integrator is switched from the input
signal to reference capacitor C22. Internal switches
connect capacitor C22 to the integrator input so that
its polarity is opposite that of the previously applied
input signal. This causes the integrator to discharge
back towards zero. The number of clock pulses
counted between the beginning of this cycle and the
time when the integrator output passes through zero
is a digital measure of the magnitude of the input
signal. This count is stored in an internal latch on U6
for output to the microcomputer.
4.23.4 ZERO INTEGRATOR PHASE
One minor additional phase is included to insure that
the integration capacitor C20 is fully discharged to
zero volts. This typically lasts 100-200 counts.
4.24 CLOCK CIRCUIT
A clock is required for the A/D converter, integrated
circuit U6. The clock signal is generated internally in
microcomputer U7 and appears on port pin “P1.0”.
The frequency is internally set by the
microcomputer’s software and is nominally 120 KHZ.
4.25 POWER SWITCHING, VOLTAGE
REGULATION AND SUPPORT CIRCUITRY
Additional circuitry is included to switch the battery
supply, provide voltage regulation and detect low
battery voltage conditions.
4.25.1 BATTERY SWITCHING
In order to conserve battery life, the battery supply is
switched on and off as needed by the scale.
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