ADE7758_15 datasheet(54/72 Pages) AD

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ADE7758

Data Sheet

Rev. E | Page 54 of 72

0.088

MHz

54.64

0.161

−

AWATTOS

The low-pass filter used to obtain the rms measurements is not

ideal; therefore, it is recommended to synchronize the readings

with the zero crossings of the voltage waveform and to average a

few measurements when reading the rms registers.

Calibration of IRMS and VRMS Offset

IRMSOS and VRMSOS are used to cancel noise and offset

contributions from the inputs. The calibration method is the

same whether calibrating using the pulse outputs or line

accumulation. Reading the registers is required for this

calibration because there is no rms pulse output. The rms offset

calibration should be performed before VAGAIN calibration.

The rms offset calibration also removes offset from the VA

calculation. For this reason, no VA offset register exists in the

ADE7758.

The ADE7758 IRMS measurement is linear over a 500:1 range,

and the VRMS measurement is linear over a 20:1 range. To

measure the voltage VRMS offset (xVRMSOS), measure rms

values at two different nonzero current levels, for example,

VNOM and VFULLSCALE/20.

To measure the current rms offset (IRMSOS), measure rms

values at two different nonzero current levels, for example, ITEST

and IFULLSCALE/500. This translates to two test conditions: ITEST

and VNOM, and IFULLSCALE/500 and VFULLSCALE/20. Figure 85 shows

a flowchart for calibrating the rms measurements.

STEP 1

SET

CONFIGURATION

REGISTERS FOR

ZERO CROSSING

ON ALL PHASES

STEP 2

SET INTERRUPT

MASK FOR

ZERO CROSSING

ON ALL PHASES

STEP 3

STEP 4

READ RMS

REGISTERS

STEP 5

WRITE TO

xVRMSOS

xIRMSOS

SET UP

SYSTEM FOR

ITEST,VNOM

SET UP SYSTEM

FOR IFULLSCALE/500,

VFULLSCALE/20

START

TESTED

ALL

PHASES?

YES

TESTED

ALL

CONDITIONS?

STEP 4A

CHOOSE N

n= 0

STEP 4D

READ

xIRMS

xVRMS

STEP 4E

CALCULATE THE

AVERAGE OF N

SAMPLES

STEP 4B

RESET INTERRUPT

STATUS REGISTER

END

n=n+1

n= N?

YES

STEP 4C

INTERRUPT?

Figure 85. RMS Calibration Routine

ADE7758 Datasheet(PDF) 54 Page - Analog Devices

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