Datasheet

ADE7758 Data Sheet
Rev. E | Page 54 of 72
00.0882
MHz1054.64
40.161
29
=−=×
×
×
=
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,
V
NOM
and V
FULLSCALE
/20.
To measure the current rms offset (IRMSOS), measure rms
values at two different nonzero current levels, for example, I
TEST
and I
FULLSCALE
/500. This translates to two test conditions: I
TEST
and V
NOM
, and I
FULLSCALE
/500 and V
FULLSCALE
/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
I
TEST
,V
NOM
SET UP SYSTEM
FOR I
FULLSCALE
/500,
V
FULLSCALE
/20
START
TESTED
ALL
PHASES?
YES NO
TESTED
ALL
CONDITIONS?
1
2
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?
NO YES
YES NO
STEP 4C
INTERRUPT?
04443-084
Figure 85. RMS Calibration Routine