MCP3918 datasheet(29/88 Pages) MICROCHIP

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DS20005287A-page 29

MCP3918

4.22

Measurement Error

The measurement error specification is typically used

in power metering applications. This specification is a

measurement of the linearity of the active energy of a

given power meter across its dynamic range.

For this measurement, the goal is to measure the

active energy of one phase when the voltage Root

Mean Square (RMS) value is fixed, and the current

RMS value is sweeping across the dynamic range

specified by the meter. The measurement error is the

non-linearity error of the energy power across the

current dynamic range. It is expressed in percent (%).

Equation 4-12 shows the formula that calculates the

measurement error:

EQUATION 4-12:

In the present device, the calculation of the active

energy is done externally, as a post-processing step

that

typically

happens

the

microcontroller,

considering, for example, one ADC as current channel

and the other MCP3918 ADC as voltage channel. The

voltage channel is fed with a full-scale sine wave at

600 mV peak and is configured with GAIN = 1 and

DITHER = Maximum. To obtain the active energy

measurement error graphs, the current channel is fed

with sine waves with amplitudes that vary from 600 mV

peak to 60 µV peak, representing a 10,000:1 dynamic

range. The offset is removed on both current and

voltage channels, and the channels are multiplied

together to give instantaneous power. The active

energy is calculated by multiplying the current and

voltage channel, and averaging the results of this

power during 20 seconds, to extract the active energy.

The sampling frequency is chosen as a multiple integer

of line frequency (coherent sampling). Therefore, the

calculation does not take into account any residue

coming from bad synchronization.

The measurement error is a function of IRMS, varies

with the OSR, averaging time, MCLK frequency and is

tightly

coupled

with

the

noise

and

linearity

specifications. The measurement error is a function of

the linearity and THD of the ADC, while the standard

deviation of the measurement error is a function of the

noise specification of the ADC. Overall, the low THD

specification enables low measurement error on a very

large dynamic range (e.g. 10,000:1). A low noise and

high SNR specification enables the decrease of the

measurement time and, therefore, of the calibration

time, to obtain a reliable measurement error

specification.

Figure 2-5 shows the typical measurement error

curves obtained with the samples acquired by the

MCP3918, using the default settings with 1-point and

2-point calibration. These calibrations are detailed in

Section 8.6 “Energy Measurement Error Consider-

ations”

Measurement Error I

RMS

 Measured Active Energy Active Energy present at inputs

–

Active Energy present at inputs

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100%

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