AN780

© 2002 Microchip Technology, Inc.

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

µF 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

µS. 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.

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:

1024

N = 256

–

= 256 –

1.0703MHz x 200mS

1024

= 46.957

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