Cirrus Logic CS5460A Manual Download Page 47

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CS5460A

DS284PP4

the current-limit protection resistors (mentioned
earlier).

Before determining a typical set of values for R

V+

V-

, C

V+

, C

V-

, C

Vdiff

, R

I+

, R

I-

, C

I+

, C

I-

, and C

Idiff

Figure 7, several other factors should be consid-
ered:

1. Values for the above resistors/capacitors should
be chosen with the desired differential-mode (and
common-mode) lowpass cutoff frequencies in
mind. In general, the differential cutoff frequen-
cies should not be less than 10 times the cut-off fre-
quencies of the internal voltage/current channel
filters, which can be estimated by studying
Figure 4 and Figure 5. From these figures, we see
that the internal voltage channel cutoff frequency is
at ~1400 Hz while the current channel cutoff fre-
quency is at ~1600 Hz. If the cutoff frequency of
the external anti-aliasing filter is much less than
10x these values (14000 Hz and 16000 Hz), then
some of the harmonic content that may be present
in the voltage/current signals will be attenuated by
the voltage/current channel input anti-aliasing fil-
ters, because such R-C filters will begin to roll off
at a frequency of 1/10th of the filter’s -3dB cutoff
frequency. If the designer is not interested in meter-
ing energy that may be present in the higher har-
monics (with respect to the fundamental power line
frequency) then the differential-mode cutoff fre-
quencies on the voltage/current input networks can
be reduced. However, relaxing the metering band-
width is usually unacceptable, as most modern
power meters are required to register energy out to
the 11th harmonic (at a minimum).

The first-order time-constants of the overall

voltage and current channel sensor networks
should be set such that they are equal (within rea-
son), or at least close in magnitude.

If the

time-constants of the voltage/current sensor net-
works are not well-matched, then the phase rela-
tionship

between

the

voltage-sense

and

current-sense signals will suffer an undesirable

shift. In this situation, the real (true) power/energy
measurements reported by the CS5460A can con-
tain significant error, because the power factor of
the sensed voltage and current signals will be sig-
nificantly different than the actual power factor of
the power line voltage/current waveforms.

Note also that in addition to the time-constants of
the input R-C filters, the phase-shifting properties
of the voltage/current sensors devices may also
contribute to the overall time-constants of the volt-
age/current input sensor networks. For example,
current-sense transformers and potential trans-
formers can impose phase-shifts on the sensed cur-
rent/voltage waveforms. Therefore, this possible
source of additional phase-shift caused by sensor
devices may also need to be considered as the de-
signer selects the final R and C values for the volt-
age/current anti-aliasing filters.

The designer

should also note that as an alternative to, or in ad-
dition to the fine adjustment of the R and C values
of the two anti-alias filters, the designer may also
be able to adjust the CS5460A’s phase compensa-
tion bits (see Phase Compensation), in order to
more closely match the overall time-constants of
the voltage/current input networks. But regardless
of whether the phase compensation bits are or are
not used to help more closely match the time-con-
stants, this requirement of equal time-constants
must ultimately be considered when the designer
selects the final R and C values that will be used for
the input filters. (Of course, this factor may not
turn out to be so important if the designer is confi-
dent that the mis-match between the voltage/cur-
rent channel time-constants will not cause enough
error to violate the accuracy requirements for the
given power/energy metering application.)