DocID026176 Rev 1

5/20

AN4470

Calibration principles and underlying theory

20

Neglecting term

εv*εi, the measured active power is

Equation 6

1.2

Accuracy and stability influence factors

All components, which have some influence on system accuracy and stability, can be found

in the input analog section.

Only a limited number of internal components determines system accuracy:

•

Voltage and current sensors

•

Signal conditioning section

•

Oscillator frequency

•

Internal reference voltage source

•

Analog-to-digital converter gain

To reach the desired stability and linearity, high quality components have to be used.

Moreover, the circuit has to be carefully designed to minimize some issues such as: short-

time repeatability, linearity or immunity degrade.

Besides, external influences can affect meter accuracy, such as:

•

Capacitive and inductive coupling to inputs and between phases (crosstalk)

•

High frequency electrical and magnetic fields (EMC)

•

Common-mode voltage between inputs and to earth

•

Low frequency magnetic fields

•

Measuring setup (wiring, earth connection ground loop)

•

Source (stability of V, I,

φ, signal quality)

•

Long-time drift

•

Humidity

Undesired external influences should be reduced to minimum through the shielding of the

analog part or compensated in hardware or software.

If system is not immune to external influences, it can only work under very special

conditions and results cannot be reproduced in other locations, where there may be a

different measuring setup. In this case, also statistical effects, due to noise, have higher

impact on short-time repeatability.

External influences on total system accuracy can be more important than the basic specified

error.

Note:

The STPM3x does not introduce any crosstalk error neither between voltage and current

inputs nor among different phases. However, the voltage front-end handles considerable

amplitude voltages, which make it a potential source of noise. Disturbances could be readily

emitted into current measurement circuitry, interfering with the signal to be measured.

Typically, this shows a non-linear error at small signal amplitudes and non-unity power

factors. At unity power factor, voltage and current signals are in phase, and crosstalk

between voltage and current channels appears as a gain error, which can be calibrated.

P'

VI

1

ε

v

ε

i

++

()

ϕ θ

+

()

cos

⋅

P

off

+

⋅⋅

=