AN780 MICROCHIP [Microchip Technology], AN780 Datasheet - Page 4

• Manufacturer Part Number: AN780
• Description: 15-Kilogram Scale Using the TC500A and the TC520
• Manufacturer: MICROCHIP [Microchip Technology]

DS00780A-page 4
AUTO-ZERO AND REFERENCE CAPACITORS
The voltage on these capacitors stay very constant so dielectric
absorption is not a consideration. The long integration time does
require capacitors with very low leakage. A .68 F polyester
capacitor was used in both cases.
TC520 TIMING
A 200ms integration time is already selected. There are a few
options available with the TC520 to do this. The exact crystal (or
clock rate) can be select in conjunction with one of the two default
timings in the TC520 or, the microprocessor can be used to
program the TC520 for the proper timing with some arbitrary crystal
frequency. The main constraint is that the TC500A has a comparator
delay of about 4 S. Also, the TC520 has a divide-by-4 on the clock
input. This means that anything around 1MHz will be acceptable.
The TC520 can be programmed by the micro to set the actual
integration time to within approximately .5ms. The crystal used in
this application is 1.0703MHz.
AN780
There are 4 clocks/count in the TC520 and the base integration
counter is 256 counts. This calculates to a timebase period of
0.9567mS with the crystal being used. The 200mS integration time
requires 209 timebase periods. Since the TC520 gives 256 timebase
periods, 47 of them need to be taken away. The value can be
determined from the equation:
The micro was programmed to load a "47" (2F
the start of the program. This will cause the TC500A to have an
integration time of 199.96mS. This value will give at least 120dB of
rejection at 50/60Hz.
The TC520 will also use the integration timing for the TC500A's
Auto-Zero phase. A 17-bit conversion will require a deintegration
time which is a function of the oscillator frequency, i.e.,
2
17
N = 256
x 4
f
OSC
–
f
OSC
= 490mS.
1024
x T
INT
= 256 – 1.0703MHz x 200mS
© 2002 Microchip Technology, Inc.
1024
H
) into the TC520 at
= 46.957