I have seen 100s of thousands systems manufactured with
AT89C51 and 7135 as weighing systems. They are everywhere in
grocery shops and in industries controlling the hoppers and
indicating weigh bridges. The original design had 555 as a
clock generator of 120KHz and another 555 as 0.1s clock for
display refresh interrupt generator. These systems dont seem
to have any clock biting.
Maybe "chopper stabilized OP-amp" had some special problem.