Datasheet

Theory of operation STPMC1
40/77 Doc ID 15728 Rev 7
9.16 Calibration (configuration bits: PM, TCS, CIX, CVX, CCA,
CCB
, CPX)
9.16.1 Voltage and current channels calibration
The 8-bit calibration values CVX and CIX (where X stands for N, R, S or T) are used as
static data for the channel ΔΣ calibrators, multiplying their streams to the following factor:
K
X
= (4096 - 1024 + 4CXX)/4096 (± 12.5 %)
When configuration bit PM
is set, a 2-bit CvX or CiX is appended to each CVX or CIX
respectively:
K
X
= (8192 - 1024 + 4CXX + CxX)/8192 (± 6.25 %)
CvX bits are part of the CCA
configuration byte while CiX are part of CCB configuration
byte.
9.16.2 Phase compensation
The STPMC1 does not introduce any phase shift between voltage and current channel.
However, the voltage and current signals come from transducers, which could have inherent
phase errors. For example, a phase error of 0.1° to 0.3° is not uncommon for a current
transformer (CT). These phase errors can vary from part to part, and they must be corrected
in order to perform accurate power calculations. The errors associated with phase mismatch
are particularly noticeable at low power factors.
The STPMC1 provides a means of digitally calibrating these small phase errors introducing
some delay. The amount of phase compensation can be set per each phase using the 4 bits
of the phase calibration configurators (CPR
, CPS, CPT).
A vector method of phase shift compensation is implemented.
The compensating voltage vector, which is produced from a frequency compensated signal
of integrated voltage vector multiplied by a given compensation constant per each phase
and is almost perpendicular to the input voltage vector, is subtracted from the input voltage
vector at the input of the decimation filter.
Those phase compensators are merged from a common coarse part CPC
and from each
phase 4-bit phase error compensator CPX
:
CPC
[1] = 0: K
PHC
= - (16 CPC[0] + CPX)
CPC
[1] = 1: K
PHC
= (16 - CPX)
When either PM
or TCS are set, a 2-bit CpC is appended to CPC to produce the following
factor:
CPC
[1] = 0: K
PHC
= - (32 CpC + 16 CPC[0] + CPX)
CPC
[1] = 1: K
PHC
= [64 - (32 CpC + 16 CPC[0] + CPX)]
CpC bits are part of the CCA
configuration byte.
The equation for phase compensation in degree is: