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Because of this effect, care should be taken in choosing an output
rate that is close to the line frequency in the application. If
the line frequency is 50 Hz, an output update rate of 50 Hz
should not be chosen as it will significantly reduce the AD7730’s
line frequency rejection (the 50 Hz will appear as a dc effect
with only 6 dB attenuation). Choosing an output rate of 55 Hz
will result in a 6 dB—attenuated aliased frequency of 5 Hz with
only a further 25 dB attenuation based on the filter profile. This
number is based on the filter roll-off and Figure 11 can be used
as a reference by dividing the frequency scale by a factor of 4.
Choosing 57 Hz as the output rate will give better than 90 dB
attenuation of the aliased line frequency which appears as a
7 Hz signal. Similarly, multiples of the line frequency should be
avoided as the output rate because harmonics of the line frequency
will not be fully attenuated. The programmability of the
AD7730’s output rate should allow the user to readily choose an
output rate that overcomes this issue. An alternative is to use
the part in nonchop mode.
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