ST AN3398 APPLICATION NOTE
AN3398
Application note
Fast digital calibration procedure for
STPMC1 based energy meters
Introduction
The STPMC1 device functions as an energy calculator and is an ASSP designed for
effective energy measurement in power line systems utilizing Rogowski coil, current
transformer, and Shunt or Hall current sensors. Used in combination with one or more
STPMSx ICs, it implements all the functions needed in a 1, 2, or 3-phase energy meter.
Due to its internal structure and features, STPMC1 allows a more effective and innovative
calibration procedure, which is explained in this document.
Advantages of this procedure are:
■ reduced calibration time
■ no need for re-calibration (calibration parameter can be written in a permanent way).
For further information about the device please refer to the STPMC1 datasheet.
This application note integrates the AN2299 application note for the STPMC1 metering chip.
Sections 1, 2, and 3.1 of the AN2299 application note can be considered valid also for the
STPMC1 device, sharing the same architecture as STPM01 and STPM10 devices, whilst
the calibration calculations shown in section 3.2 of AN2299 and in this document are slightly
different and are reported below.
November 2011 Doc ID 018812 Rev 1 1/11
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Contents AN3398
Contents
1 Calibration flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Calibration procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Working point setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Algorithm choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 Offline parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.4 Online procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.5 Coherency check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2/11 Doc ID 018812 Rev 1
AN3398 Calibration flow chart
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1 Calibration flow chart
The calibration procedure can be summarized in the following steps, which are examined in
Section 1.1.
Figure 1. Calibration flow chart
Start
Start
Start
Start
Working point
Working point
Working point
Working point
R1 or SV
R1 or SV
R1 or SV
R1 or SV
1. Working point setting
2. Algorithm choice
Offline
Offline
Offline
Offline
Online
Online
Online
Online
Decrease LED frequency
K?
K?
K?
K?
End
End
End
End
1.1 Calibration procedure
1.1.1 Working point setting
According to the information contained in AN2299, the STPMC1 device can also be
calibrated in a single point for each phase.
3. Offline parameters calculations
4. Online procedure (for each phase)
5. Coherency check (for each phase)
AM09391v1
Therefore, voltage and current nominal values of the selected phase must be defined before
running the calibration procedure, for example:
Doc ID 018812 Rev 1 3/11
Calibration flow chart AN3398
Table 1. Working point setting
Parameter Value Description
Vn 230 V Phase to neutral RMS voltage
In 5 A Phase RMS current
The other parameters, which follow, and the constants of the STPMC1 metering device (and
relative tolerances) are also known:
Table 2. STPMC1 internal parameters
Parameter Value Description
Kint_comp 1.004 Gain of decimation filter
0.875 Voltage calibrator ideal value if PM = 0
K
V
0.9375 Voltage calibrator ideal value if PM = 1
0.875 Current calibrator ideal value if PM = 0
K
I
0.9375 Current calibrator ideal value if PM = 1
len_i 2^16 Current register length
len_u 2^12 Voltage register length
π
3.14159
(1)
4 * 10^6 If oscillator frequency is 4.000 or 8.000 MHz
FM
2^22 If oscillator frequency is 4.194 or 8.388 MHz
4915200 If oscillator frequency is 4.915 or 9.830 MHz
D
UD
2^17 Internal parameter
Vref 1.23 Internal voltage reference
4 Amplification of voltage ADC for STPMS1
Au
2 Amplification of voltage ADC for STPMS2
8
Amplification of current ADC for STPMS1
32
Ai
2
Amplification of current ADC for STPMS2
16
0.815 Gain of integrator @ line frequency = 50 Hz
Kint
0.679 Gain of integrator @ line frequency = 60 Hz
0.6135 Gain of differentiator @ line frequency = 50 Hz
Kdif
0.7359 Gain of differentiator @ line frequency = 60 Hz
1. PM is CFG 21, which sets the meter precision (Class 1 or Class 0.1).
4/11 Doc ID 018812 Rev 1
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