Cirrus Logic CS5460A, Manual

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CS5460A
DS284PP4 39
forms that must be measured in most power sys-
tems are approximately sinusoidal in nature, the
designer/user will most likely need to set the RMS
levels of the AC gain calibration input signals such
that they will be significantly lower than the volt-
age/current channel’s maximum DC voltage input
level. This must be done in order to avoid the pos-
sibility that the peak values of the AC waveforms
that are to be measured will not register a value that
would be outside the available output code range of
the voltage/current A/D converters. For example,
on the voltage channel, if the Voltage Channel
Gain Register is set to it’s default power-on value
of 1.000... before calibration, then the largest pure
sinusoidal waveform that can be used in AC cali-
bration is one whose RMS-value is ~0.7071 of the
value of the voltage channel’s peak DC input volt-
age value of
±
250 mV. Thus the maximum value
of the input sinusoid would be ~176.78mV (rms).
But in many practical power metering situations,
the user will probably want to reduce the RMS
voltage input level of the AC gain calibration signal
even further, to allow for some over-ranging capa-
bility. A typical sinusoidal calibration value which
allows for reasonable over-range margin would be
0.6 of the voltage/current channel’s maximum in-
put voltage level. For the voltage channel, such a
sine-wave would have a value of 0.6 x 250mV =
150mV (rms).
For the offset calibrations, there is no difference
between the AC and DC calibration signals that
must be supplied by the user: The user should sim-
ply connect the “+” and “-’ pins of the voltage/cur-
rent channels to their ground reference level. (See
Figure 19.)
The user should not try to run both an offset and
gain calibration at the same time. This will cause
undesirable calibration results.
4.8.7 Description of Calibration Algorithms
The computational flow of the CS5460A’s AC and
DC gain/offset calibration sequences are illustrated
in Figure 20. This figure applies to both the volt-
age channel and the current channel. The following
descriptions of calibration sequences will focus on
the voltage channel, but apply equally to the cur-
rent channel.
Note: For proper calibration, it is assumed that the
value of the Voltage-/Current-Channel Gain
Registers are set to default (1.0) before running
the gain calibration(s), and the value in the
Voltage-/Current Channel AC and DC Offset
Registers is set to default (0) before running
calibrations. This can be accomplished by a
software or hardware reset of the device. The
values in the voltage/current calibration
registers do affect the results of the calibration
sequences.
4.8.7.1 AC Offset Calibration Sequence
The idea of the AC offset calibration is to obtain an
offset value that reflects the square of the RMS out-
put level when the inputs are grounded. During
normal operation, when the CS5460A is calculat-
ing the latest result for the RMS Voltage Register,
this AC offset register value will be subtracted
from the square of each successive voltage sample
in order to nullify the AC offset that may be inher-
ent in the voltage-channel signal path. Note that
+
-
XGAIN
+
-
External
Connections
+
-
AIN+
AIN-
CM +-
Full Scale
(DC or AC)
Figure 18. System Calibration of Gain.
+
-
XGAIN
+
-
External
Connections
0V
+
-
AIN+
AIN-
CM +
-
Figure 19. System Calibration of Offset.