AD7714: ALE pin as a master clock

Document created by analog-archivist Employee on Feb 23, 2016
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I am using an AD7714 with a 8051 micro.
I am currently clocking it with 2MHz from a xtal oscillator, but I am
wondering whether I could clock it with the 8051's ALE instead, which is
also roughly 2MHz. However, the micro skips ALE pulses when accessing
external memory, i.e. the ALE signal would be somewhat irregular. Would this
affect the conversion results?

 

Using the ALE pin as the master clock for the ADC is very bad idea, continue
using the crystal. Even if the ALE pin did not skip pulses, I suspect that the
ALE has very poor frequency stability over time and temperature.

The master clock is used to determine the sampling clock, the output update
rate and (most importantly in this case) the profile of the digital decimation
filter which includes the bandwidth and notch position. The AD7714 is not
tested or characterized for use with a discontinuous clock but the most likely
effects would be an increase in noise and unpredictable gain and offset errors.

If you intention is to reduce cost and you can accept a certain amount of drift
with temperature and time, then consider using a ceramic resonator.

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