CS5460A
voltage/current channel input anti-aliasing filters.
These resistors also serve as the current-limit pro-
tection resistors (mentioned earlier).
3.13 Input Filtering
Figure 6 shows how the analog inputs can be con-
nected for a single-ended input configuration. Note
here that the Vin- and Iin- input pins are held at a
constant DC common-mode level, and the varia-
tion of the differential input signal occurs only on
the Vin+ and Vin- pins. The common-mode level
on the Vin-/Iin- pins is often set at (or very near) the
CS5460A’s common-mode ground reference po-
tential. (The common-mode ground reference po-
tential is defined by the voltage at the VA- pin.)
But this is not required--the DC reference level of
the Vin-/Iin- pins can be set to any potential be-
tween [VA-] and [(VA+) - 250 mV]. In Figure 6, ob-
serve the circuitry which has been placed in front
of the current channel input pins, as one example.
The anti-aliasing filter can be constructed by calcu-
lating appropriate values for RI+ = RI-, CIdiff, and
CI+ = CI-. The sensor voltage that is created by the
voltage drop across RSHUNT is fed into the Iin+ pin,
while the voltage at the Iin- pin is held constant.
Before determining a typical set of values for RV+,
RV-, CV+, CV-, CVdiff, RI+, RI-, CI+, CI-, and CIdiff in
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 frequencies
should not be less than 10 times the cut-off fre-
quencies of the internal voltage/current channel fil-
ters, which can be estimated by studying Figure 4
and Figure 5. In these figures, the internal voltage
channel cutoff frequency is ~1400 Hz while the
current channel cutoff frequency is ~1600 Hz. If
the cutoff frequency of the external anti-aliasing fil-
ter 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 filters, because such R-C filters
will begin to roll off at a frequency of 1/10th of the
filter’s -3 dB cutoff frequency. If the designer is not
interested in metering energy that may be present
in the higher harmonics (with respect to the funda-
mental power line frequency) then the differen-
tial-mode cutoff frequencies on the voltage/current
input networks can be reduced. However, relaxing
the metering bandwidth is usually unacceptable,
as most modern power meters are required to reg-
ister energy out to the 11th harmonic (at a mini-
mum).
Figure 7 shows a differential bipolar input configu-
ration. Note in Figure 7 that the “+” and “-” input
pins for the voltage/current channels are equally
referenced above and below the CS5460A’s
ground reference voltage. Such a differential bipo-
lar input configuration can be used because the
CS5460A voltage/current channel inputs are able
to accept input voltage levels as low as -250 mV
(common-mode) below the VA- pin ground refer-
ence, which is defined by the voltage at the VA-
pin. (In fact, if desired, the center-tapped reference
of these differential input pairs could be connected
to a DC voltage of, for example, +2 V, because
+2 V is within the available common-mode range
of [VA-] and [VA+ - 250mV]. But this configuration
may not be practical for most metering applica-
tions.) In the differential bipolar input configuration,
the voltage signals at the Vin- and Iin- pins will fluc-
tuate in similar fashion to the Vin+/Iin+ pins, except
the voltages at the “-” pins will be 180 degrees out
of phase with respect to the voltage signals at the
“+” pins. Therefore the signal paths to the “+” and
“-” pins play an equal role in defining the differential
voltage input signal. Because of this, the protection
resistors placed on Vin-/Iin- pins will play an equal-
ly important role as the resistors on the Vin+/Iin+
pins, in defining the differential responses of the
2. The first-order time-constants of the overall volt-
age and current channel sensor networks should
be set such that they are equal (within reason), or
at least close in magnitude. If the time-constants of
the voltage/current sensor networks are not
well-matched, then the phase relationship be-
tween the voltage-sense and current-sense sig-
nals will suffer an undesirable shift. In this
situation, the real (true) power/energy measure-
ments reported by the CS5460A can contain signif-
icant error, because the power factor of the sensed
voltage and current signals will be significantly dif-
ferent than the actual power factor of the power
line voltage/current waveforms.
34
CIRRUS [ CIRRUS LOGIC ]
