CS5460A
48
DS284PP4
channel is usually high enough to be ignored, the
impedance across the current channel inputs may
need to be taken into account by the designer when
the desired cutoff frequencies of the filters (and the
time-constants of the overall input networks) are
computed. Also, because of this rather low input
impedance across the current channel inputs, the
designer should note that the as the values for R
I+
and/or R
I-
are increased, the interaction of the cur-
rent channel’s input impedance can begin to cause
a significant voltage drop situation within the cur-
rent channel input network. If this is not taken into
account, values may be chosen for R
I+
= R
I-
that are
large enough to cause a discrepancy between the
expected (theoretical) sensor gain and the actual
sensor gain of the current sensor network, which
may not be anticipated by the designer. Also, if this
voltage drop effect is not considered, the designer
may select values for R
I+
and R
I
that are slightly
larger than they should be, in terms of maximizing
the available dynamic range of the current channel
input. And for the very same reason, the line-cur-
rent-to-sensor-output-voltage conversion factor of
the current sensor may not be optimized if this volt-
age division is not considered by the designer,
when (for example) the designer is selecting a val-
ue for the burden resistor for a given current trans-
former. This issue should be considered, although
the user should note that a slight voltage drop only
causes a slight loss in available dynamic range, and
the effects of this voltage drop on the actual current
channel sensor gain can be removed during gain
calibration of the current channel.
4. Referring to Figures 6 - 9, the designer may de-
cide to use only some of the capacitors/resistors
shown in these example circuit diagrams. Howev-
er, note that the all of the filter capacitors C
V+
, C
V-
,
C
I+
, C
I-
, C
Vdiff
, and C
Idiff
can, in some situations,
help to improve the ability of both input networks
to attenuate very high-frequency RFI that can enter
into the CS5460A’s analog input pins. Therefore,
during layout of the PCB, these capacitors should
be placed in close proximity to their respective in-
put pins.
If any/all of the common-mode connected capaci-
tors (C
V+
, C
V-
, C
I+
, C
I-
) are included in the input
networks, their values should be selected such that
they are at least one order of magnitude smaller
than the value of the differential capacitors (C
Vdiff
,
and C
Idiff
). This is done for at least two reasons:
a) The value tolerance for most types of commer-
cially available surface-mount capacitors is not
small enough to insure appreciable value matching,
between the value of C
V+
vs. C
V-
, as well as be-
tween the value of C
V+
vs. C
V-
. Such value mis-
match
can
adversely
affect
the
desired
differential-mode response of the voltage/current
input networks. By keeping the values of these
common-mode capacitors small, and allowing the
value of the C
Vdiff
, and C
Idiff
to dominate the differ-
ential 1st-order time-constant of the input filter net-
works, this undesirable possibility of frequency
response variation can be minimized.
b) The common-mode rejection performance of the
CS5460A is very good within the frequency range
over which the CS5460A performs A/D conver-
sions. Addition of such common-mode caps can
actually often degrade the common-mode rejection
performance of the entire voltage/current input net-
works. Therefore, choosing relatively small values
for (C
V+
, C
V-
, C
I+
, C
I-
) will provide necessary com-
mon-mode rejection at the much higher frequen-
cies, and this will also allow the CS5460A to
realize its relatively good CMRR performance in
the frequency-range of interest.
We start with the current channel. [Note that for
the purposes of this discussion, we assume that the
phase-shifts caused by the voltage-sense trans-
former and current-sense transformer in Figure 7
are negligible (or even equal), although such an as-
sumption should definitely not be made in a re-
al-life practical meter design situation.]
Using
commonly available values for our components, if
Summary of Contents for CS5460A
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