AN780 Datasheet, PDF (1/10 Pages) Microchip Technology – 15-Kilogram Scale Using the TC500A and the TC520

AN780
15-Kilogram Scale Using the TC500A and the TC520
Author: Ted Dabney,
Microchip Technology, Inc.
INTRODUCTION
A 15kg weighing scale was designed using Microchip's TC500A
Analog Processor and the TC520 16-bit Controller. The scale is
required to resolve down to 1/8 gram and correct to within 61/2
gram. This project takes into account all aspects of a functional scale:
•
Dynamic Range
•
Strain Gauge Compensation
•
Zeroing
•
Oversampling
•
Units Conversion (kilograms to pounds)
The TC500A is an analog processor device which performs a dual-
slope analog-to-digital conversion function. All of the counting and
timing for the conversion must be controlled by an external source.
In nearly all applications, this control source is a microprocessor.
The microprocessor is programmed to monitor the status and to
control the timing of the TC500A. It must also be programmed to
count the conversion results.
The TC500A has no agenda of its own so it can be used to generate
slow, high resolution conversions or fast, low resolution conver-
sions. The trade-off for accuracy is about 1000 counts per millisec-
ond of integration time, i.e., 16-bits with T_INT approximately equal
to 60mS. Typically, the total conversion time is about 4 times the
integration time but, with the TC500A, this is quite flexible.
T_INT = C_INT R_INT V_INT/V_IN (max)
eq1
The TC520 is a digital interface device which can be used to
replace all of the TC500A timing and counting functions performed
by a microprocessor. The TC520 can use either a crystal or
external clock as a time-base to control the operation of either a
TC500 or a TC500A.
C_REF
R_INT
C_INT
Control Logic
A B Converter State
0 0 Zero Integrator Output
0 1 Auto-zero
1 0 Signal Integrate
1 1 Deintegrate
C_R⁺_EF
V+_REF
79
C_AZ
V_R⁺_{EF 8 6 CR–EF 4 BUF 3 CAZ}
1 ^CINT
V+_IN ^{11 SWI}
SW_R SW_R
SW_R^–_I SW+_RI
Buffer
–
+
Integrator
–
+
Comp 1
+
–
TC500/TC500A
Comp 2
–
+
Level
Shift
14 Comp
Output
Analog
Common
SW_Z
5
V_I^–_N ^{10 SWI}
SW+_RI SW_R^–_I
SW₁
SW_IZ ^SWZ
Analog
Switch
Control
Signals
Polarity
Detection
Phase
Decoding
Logic
15 _GND
2
16
V_S^– V_S⁺
12 13
A
B
Control Logic
FIGURE 1: Functional block diagram.
© 2002 Microchip Technology, Inc.
DS00780A-page 1