Cirrus Logic CS5460A, Руководство пользователя

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CS5460A
46 DS284PP4
through the primaries of the current-sense trans-
former. This can in turn generate as much as 10V
across the secondaries of the current-sense trans-
former. This voltage is high enough to turn on one
or more of the internal protection diodes located off
of the Iin+/Iin- pins. Therefore, the value of the
protection resistor that will limit the current flow to
less than 100 mA would be (10V - 7V) / 100 mA =
30
Ω
. In order to provide some margin and to use
the same resistor values that are used on the
Vin+/Vin- pins, we could use 470
Ω
as a lower limit
for the R
I+
and R
I-
resistors shown in Figure 7.
Referring to the circuit implementations shown in
Figures 6, 8, and 9, note that when resistor-divider
configurations are used to provide the voltage
channel sense voltage, the VIN+ pin does not need
an additional, separate, dedicated protection resis-
tor. This is because the resistive voltage-divider
already provides the series resistance that is needed
for this protection resistance (from R
1
and R
2
).
(And note in Figure 8 that this is true for both the
VIN+ pin and the VIN- pin.) In Figure 7, a voltage
transformer is used as the voltage sensor. When
any type of transformer is used as the sensor device
for voltage (or current) channel, a dedicated protec-
tion resistor R
V+
should be installed in series with
the VIN+ pin, and similarly, a resistor (R
V-
) should
be installed in series to the VIN- input pin.
Additional considerations/techniques regarding the
protection of the analog input pins against sudden
high-frequency, high-level voltage/current surges
are discussed in Section 4.14.
4.13 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 variation
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+) - 250mV]. In Figure 6,
observe 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 R
I+
= R
I-
, C
Idiff
, and C
I+
=
C
I-
. The sensor voltage that is created by the volt-
age drop across R
SHUNT
is fed into the Iin+ pin,
while the voltage at the Iin- pin is held constant.
Figure 7 shows a differential bipolar input config-
uration. 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, +2V, b2V
is within the available common-mode range of
[VA-] and [VA+ - 250mV]. But this configuration
may not be so practical for most metering applica-
tions.) In the differential bipolar input configura-
tion, the voltage signals at the Vin- and Iin- pins
will fluctuate 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 sig-
nals 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 equally important role as the resis-
tors on the Vin+/Iin+ pins, in defining the differen-
tial responses of the voltage/current channel input
anti-aliasing filters. These resistors also serve as