STPM32 datasheet(82/121 Pages) STMICROELECTRONICS

82 / 121 page

Application design and calibration

STPM32, STPM33, STPM34

82/121

DS10272 Rev 8

Referring to Section 9.1 and Section 5 on page 24, having R1 or kS calculated as stated in

the previous section, the target values of voltage and current RMS registers, XV and XI

respectively are calculated as follows:

Note:

For the above calculation, the calculated value of the component kS or R1 (according to the

chosen design method) must be used; the difference of the real component is compensated

by calibration as a tolerance.

To start calibration, the device has to be programmed with the proper gain and current

sensor; moreover, to obtain the greatest correction dynamic, calibrators are initially set in

the middle of their range (0x800), thus obtaining a calibration range of ± 12.5% per voltage

or current channel.

After applying VN and current IN to the meter, a certain number of voltage and current RMS

samples must be read and averaged (please, refer to averaged register values as VAV and

IAV) to calculate voltage and channel calibrators as follows:

The above procedure must be repeated for all voltage/current channels.

9.2.2

Phase calibration (PHVx, PHCx bits)

The STPM3x does not introduce any phase shift between voltage and current channels.

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 phase compensation block provides a method of digital phase correction of the phase

shifting between voltage and current channels which can be introduced by the external

component intrinsic characteristics or by external component mismatch. The amount of

Table 37. Calibration target values

Parameter

Value

Voltage register value at VN

Current register value at IN

VN AV calV 2





Vref 1R1 R2







------------------------------------------------

IN AI calI kS 2







Vref

-----------------------------------------------------

Table 38. Calibrator calculation

Parameter

Value

Calibrator value

Correction factor

CHV

14336

VAV

---------- 12288

–



CHC

14336

IAV

-------- 12288

–



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