Cirrus Logic AN366 - Manual (Page 10)

AN366
10 AN366REV2
Current Scale Register
To perform calibration with less than full scale load without using the above procedure, it is possible to set
the current channel's Scale register. The current channel calibration data path contains a Scale register
(page 18, address 63) that can be adjusted before calibration to accommodate the non-full-scale load.
where:
I
SCALE
Value stored in the Scale register before calibration
I
MAX
Maximum current of the meter defined by the customer
I
REF
Load current before calibration, as measured with a reference meter, assuming stable
input
Follow the steps below to set the current channel’s Scale register.
1. Set the load current, I
REF
(assuming V
REF
is set to full scale).
2. Confirm that the reference meter shows V
REF
and I
REF
of the input.
3. Set the Scale register per Equation 8.
4. Send the calibration command.
5. After calibration, the meter is adjusted for a full-scale voltage of V
MAX
and I
MAX
and will currently be
measuring the V
REF
and I
REF
measurements.
6. The Scale register is not in the normal data path but instead in the calibration path.
4.1.3 AC Offset Calibration
Following gain calibration, there may still be some AC offset remaining. AC offset calibration will allow for
the removal of the remaining offset. The AC offset effects are only applicable to the I
RMS
registers at small
input. The AC offset calibration only needs to be performed when I
RMS
readings are required to span a large
dynamic range with high accuracy.
4.2 Available Compensations
Three compensations are available in the CS5480/84/90: phase, no-load active power, and no-load reactive
power offset.
4.2.1 Phase Compensation
Phase compensation adjusts phase mismatches between the voltage and current channels. Setting the cur-
rent to lag the voltage by 60º (the center of the COS range of 0º - 90º) allows the system to distinguish ad-
ditional or less phase delay from the power factor (PF) directly. Follow the steps below to perform this
compensation:
1. Apply source at full scale with a 60º phase shift (PF = 0.5 lagging)
2. Start continuous convert
3. Read the PF register and calculate:
Phase error = ACOS(register PF)-60º
4. Calculate phase compensation (PC) register (MCLK=4.096MHz):
50Hz PC register = phase error/0.008789
60Hz PC register = phase error/0.010547
Phase error can be adjusted when it falls within ±8.99º at 50Hz or ±10.79º at 60Hz. Figure 6 shows the
phase offset error range. When phase error is below -4.5º at 50Hz or -5.4º at 60Hz and above 0º, it is nec-
essary to adjust both coarse compensation and fine compensation. The coarse and fine compensation set-
tings for each region are shown in Figure 6.
I
SCALE
I
REF
I
MAX
------------ 0.6  2
23
 = [Eq: 8]