Cirrus Logic CS5480 - Manual (Page 63)

CS5480
DS980F3 63
7. SYSTEM CALIBRATION
Component tolerances, residual ADC offset, and
system noise require a meter to be calibrated before it
meets a specific accuracy requirement. The CS5480
provides an on-chip calibration algorithm to operate the
system calibration quickly and easily. Benefiting from
the excellent linearity and low noise level of the
CS5480, normally a CS5480 meter only needs one
calibration at a single load point to achieve accurate
measurements over the full load range.
7.1 Calibration in General
The CS5480 provides DC offset and gain calibration
that can be applied to the instantaneous voltage and
current measurements and AC offset calibration, which
can be applied to the current RMS calculation.
Since the voltage and current channels have
independent offset and gain registers, offset and gain
calibration can be performed on any channel
independently.
The data flow of the calibration is shown in Figure 25.
Note that in Figure 25 the AC offset registers and gain
registers affect the output results differently than the DC
offset registers. The DC offset and gain values are
applied to the voltage/current signals very early in the
signal path; the DC offset register and gain register
values affect all CS5480 results. This is not true for the
AC offset correction. The AC offset registers only affect
the results of the RMS current calculation.
The CS5480 must be operating in its active state and
ready to accept valid commands. Refer to section
6.1.2
Instructions on page 29 for different calibration
commands. The value in the SampleCount register
determines the number (N) of OWR samples that are
averaged during a calibration. The calibration
procedure takes the time of N +T
SETTLE
OWR samples.
As N is increased, the calibration takes more time but
the accuracy of calibration results tends to increase.
The DRDY bit in the Status0 register will be set at the
completion of calibration commands. If an overflow
occurs during calibration, other Status0 bits may be set
as well.
7.1.1 Offset Calibration
During offset calibrations, no line voltage or current
should be applied to the meter. In other words, the
differential signal on voltage inputs VIN± or current
inputs IIN1± (IIN2±) of the CS5480 should be 0V.
7.1.1.1 DC Offset Calibration
The DC offset calibration command measures and
averages DC values read on specified voltage or
current channels at zero input and stores the inverse
result in the associated offset registers. This DC offset
will be added to instantaneous measurements in
subsequent conversions, removing the offset.
The gain register for the channel being calibrated
should be set to 1.0 prior to performing DC offset
calibration.
DC offset calibration is not required if the high-pass filter
is enabled on that channel because the DC component
will be removed by the high-pass filter.
7.1.1.2 Current Channel AC Offset Calibration
The AC offset calibration command measures the
residual RMS value 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.
V
RMS
* , I
RMS
*
Registers
IN Modulator Filter
N 
* Denotes readable/writable register
Ϯ Applies only to the current path (I1, I2)
N
 N
-1
 N
DC
RMS
-1
RMS
0.6(Scale * Ϯ )
V * , I * , P * , Q *
Registers
I
GAIN
* , V
GAIN
*
Registers
I
DCOFF
* , V
DCOFF
*
Registers
I
ACOFF
* Ϯ Register
Figure 25. Calibration Data Flow