Cirrus Logic CS5484 - Manual (Page 63)

CS5484
DS981F3 63
7.1.1.2 AC Offset Calibration
The AC offset calibration command measures the
residual RMS values on the current channel at zero
input and stores the squared result in the associated AC
offset register. This AC offset will be subtracted from
RMS measurements in subsequent conversions,
removing the AC offset on the associated current
channel.
The AC offset register for the channel being calibrated
should first be cleared prior to performing the
calibration. The high-pass filter should be enabled if AC
offset calibration is used. It is recommended that
T
SETTLE
be set to 2000 ms before performing an AC
offset calibration. Note that the AC offset register holds
the square of RMS value measured during calibration.
Therefore, it can hold a maximum RMS noise
of
. This is the maximum RMS noise that AC
offset correction can remove.
7.1.2 Gain Calibration
Prior to executing the gain calibration command, gain
registers for any path to be calibrated (Vx
GAIN
, Ix
GAIN
)
should be set to 1.0, and T
SETTLE
should be set to
2000 ms. For gain calibration, a reference signal must
be applied to the meter. During gain calibration, the
voltage RMS result register (Vx
RMS
) is divided into 0.6,
and the current RMS result register (Ix
RMS
) is divided
into the Scale register. The quotient is put into the
associated gain register. The gain calibration algorithm
attempts to adjust the gain register (Vx
GAIN
, Ix
GAIN
)
such that the voltage RMS result register (Vx
RMS
)
equals 0.6, and the current RMS result register (Ix
RMS
)
equals the Scale register.
Note that for the gain calibration, there are limitations on
choosing the reference level and the Scale register
value. Using a reference or a scale that is too large or
too small can cause register overflow during calibration
or later during normal operation. Either condition can set
Status register bits I1OR (I2OR) V1OR (V2OR). The
maximum value that the gain register can attain is four.
Using inappropriate reference levels or scale values
may also cause the CS5484 to attempt to set the gain
register higher than four, therefore the gain calibration
result will be invalid.
The Scale register is 0.6 by default. The maximum
voltage (U
MAX
Volts) and current (I
MAX
Amps) of the
meter should be used as the reference signal level if the
Scale register is 0.6. After gain calibration, 0.6 of the
Vx
RMS
(Ix
RMS
) register represents U
MAX
Volts (I
MAX
Amps) for the line voltage (load current); 0.36 of the
Px
AVG
, Qx
AVG
, or Sx register represents U
MAX
×I
MAX
Watts, Vars, or VAs for the active, reactive, or apparent
power.
If the calibration is performed with U
MAX
Volts and I
CAL
Amps and I
CAL
<I
MAX
, the Scale register needs to be
scaled down to 0.6×I
CAL
/I
MAX
before performing gain
calibration. After gain calibration, 0.6 of the Vx
RMS
register represents U
MAX
Volts, 0.6xI
CAL
/I
MAX
of the
Ix
RMS
register represents I
CAL
Amps, and
0.36 x I
CAL
/ I
MAX
of the Px
AVG
, Qx
AVG
, or Sx register
represents U
MAX
xI
CAL
Watts, Vars, or VAs.
7.1.3 Calibration Order
1. If the HPF option is enabled, then any DC
component that may be present in the selected
signal channel will be removed, and a DC offset
calibration is not required. However, if the HPF
option is disabled, the DC offset calibration should
be performed.
When using high-pass filters, it is recommended that
the DC offset register for the corresponding channel
be set to zero. Before performing DC offset calibra-
tion, the DC offset register should be set to 0, and the
corresponding gain register should be set to 1.
2. If there is an AC offset in the Ix
RMS
calculation, the
AC offset calibration should be performed on the
current channel. Before performing AC offset
calibration, the AC offset register should be set to 0.
It is recommended that T
SETTLE
be set to 2000ms
before performing an AC offset calibration.
3. Perform the gain calibration.
4. If an AC offset calibration was performed (step 2),
then the AC offset may need to be adjusted to
compensate for the change in gain (step 3). This can
be accomplished by restoring zero to the AC offset
register and then performing an AC offset calibration.
The adjustment could also be done by multiplying
the AC offset register value that was calculated in
step 2 by the gain calculated in step 3 and updating
the AC offset register with the product.
7.2 Phase Compensation
A phase compensation mechanism is provided to adjust
for meter-to-meter variation in signal path delays. Phase
offset between a voltage channel and its corresponding
current channel can be calculated by using the power
factor (PF1, PF2) register after a conversion.
1) Apply a reference voltage and current with a lagging
power factor to the meter. The reference current
waveform should lag the voltage with a 60° phase
shift.
2) Start continuous conversion.
0xFFFFFF