ADE9178 datasheet(55/122 Pages) AD

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Data Sheet

ADE9178

APPLICATIONS INFORMATION

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Rev. A | 55 of 122

CALIBRATION METHOD

The system is calibrated at nominal operating voltage and current

using an accurate source. The worked out example is shown only

for phase A. The calculation remains same for other channels as

well. All calibration steps involve providing a known signal to the

relevant ADC channels and compare one of the output registers to

the expected value of the register.

System Parameters:

►

VNOMINAL = 220 VRMS

►

INOMINAL = 10 ARMS

►

Line Frequency = 50 Hz

►

Shunt Resistor = 500 µΩ

►

Voltage Divider

►

R1 = 990 kΩ

►

R2 = 1 kΩ

►

Divider Ratio = 0.001

Theoretical Full-Scale Voltage =

VFS_T= VADC_FS

Divider_Ratio

÷ 2= 10.001 ÷ 2

707 VRMS

(37)

Note that the fully-differential VFS_T is used for this calculation. See

the Worked Examples section on how to calculate VFS_T for the

system.

Full-Scale Current =

IFS= IADC_FS

Sℎunt_Resistance

÷ 2= 0.03125

500×10−6

÷ 2=

44.188 ARMS

(38)

Gain Calibration

This section explains how to calculate the xxGAIN register, which

affects all output parameters of the ADE9178. RMS values are used

to calibrate the gain.

With the nominal voltage and current inputs, read the appropriate

RMS register (for example, AIRMS, AVRMS, or AUXRMS). It is

recommended to read the RMS values once per zero crossings for

1 sec and average them for better accuracy.

The expected RMS register value for given voltage and current can

be calculated as follows:

xRMSEXPECTED=RMSFS_CODES×XNOMINAL

XFS

(39)

where, XNOMINAL is the nominal signal applied at the ADC input and

XFS is either VFS_T, IFS, or AUXFS.

The xxGAIN register can be calculated from expected and ob-

served RMS register as follows:

xxGAIN= xRMSEXPECTED

xRMSMEASURED

−1 ×227 (40)

Example

With the given example system parameters, the following equation

shows the calculations for AV channel gain:

AVRMSEXPECTED=107310840×220

707

33392341 decimal

(41)

If AVRMSMEASURED = 33512088 decimal, then:

AVGAIN= 33392341

33512088

−1 ×227=−479595

decimal=0×FFF8AE96

(42)

DC Offset Calibration

All xOS registers must be 0 for AC metrology applications.

RMS Offset Calibration

To calibrate RMS offset register, apply a small signal typically at

2000:1 or less dynamic range to the channel the user is trying to

calibrate. It is recommended to keep the other channel as nominal.

In this example, to calibrate the current offset, the calibration

current is set to 20 mA and the voltage is kept at nominal. Read the

RMS values once per zero crossings for 1 sec and average them

for better accuracy.

The expected RMS register value for given input can be calculated

as follows:

xRMSEXPECTED=RMSFS_CODES×XREDUCED

XFS

(43)

where, XREDUCED is the reduced calibration signal applied at the

ADC input and XFS is either VFS_T, IFS, or AUXFS.

The xxRMSOS register can be calculated from expected and ob-

served RMS register as follows:

xxRMSOS= xRMSEXPECTED

2 −xRMSMEASURED

215

(44)

Example

With the reduced input current to 20 mV, the following equation

shows the calculations for AI channel offset:

AIRMSEXPECTED=107310840×0.02

44.188

48570 decimal

(45)

If AIRMSMEASURED = 48733 decimal, then:

AIRMSOS= 485702−487332

215 =

−484

decimal=0×FFFFFE1D

(46)

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