ADE9078 datasheet(37/108 Pages) AD

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ADE9078

Data Sheet

Rev. 0 | Page 36 of 107

Power-Based and Filter-Based RMS Measurement

Algorithms

Filter-Based Total RMS

The ADE9078 offers current and voltage rms measurements,

which are calculated by squaring the input signal, low-pass

filtering, and then taking the square root of the result, as shown

in Figure 54.

215

xRMSOS

xRMS

52,866,837

xV_PCF OR xI_PCF

VOLTAGE OR CURRENT

CHANNEL WAVEFORM

Figure 54. Filter Based RMS

The low-pass filter, LPF2, extracts the rms value, attenuating

harmonics of a 50 Hz or 60 Hz fundamental by at least 64 dB so

that at full scale, the variation in the calculated rms value is very

small, ±0.064% error. Note that the rms reading variation

increases as the input signal becomes smaller because the noise

in the measurement increases.

The filter based rms measurement is typically within 0.5% error

over a 5000:1 dynamic range and within 0.1% error over a 1000:1

dynamic range. Refer to the specifications in Table 1 to understand

what performance to expect from this measurement.

Note that the xRMS register does not read 0 with the xP and xN

inputs shorted together.

The filter based rms has a bandwidth of 1.6 kHz, as given in

Table 1.

The rms calculations, one for each channel, AIRMS, BIRMS,

CIRMS, NIRMS, AVRMS, BVRMS, and CVRMS, are updated

every 4 kSPS. The ISUMRMS calculation uses the same method

to calculate ISUMRMS, where ISUM = IA + IB + IC ± IN, and also

updates at 4 kSPS (see the Neutral Current RMS, RMS of Sum of

Instantaneous Currents section for more information).

The xRMS value at full scale is 52,866,837d. Table 13 shows the

rms settling time to 99% of full scale for a 50 Hz signal.

Table 13. RMS Settling Time

Configuration

RMS Settling Time, FS = 99% (sec)

Integrator On, HPF On, and

LPF2 On

1.09

Integrator Off, HPF On, and

LPF2 On

0.96

For high performance at small input signals, below 1000:1, it is

recommended to calibrate the offset of this measurement using

the xRMSOS registers. The offset must be calibrated at the smallest

input signal that requires good performance—do not calibrate

this measurement with zero input signal.

The following equation indicates how the xRMSOS register

value modifies the result in the xRMS register:

xRMSOS

xRMS0

xRMS







where xRMS0 is the initial xRMS register value before offset

calibration.

At 1000:1, the expected xRMS0 = 52,866,837/1000 =

52,866.837. Then, one bit in the xRMSOS register changes

xRMS by (52,867.147 − 52,866.837)/52,866.837 = 0.0006%.

147

867

1000

52,866,837





















xRMS

Neutral Current RMS, RMS of Sum of Instantaneous

Currents

The ADE9078 calculates the neutral current rms from a neutral

current sensor input into the INP and INN pins, and stores the

result in the NIRMS register. A NIRMSOS register allows offset

calibration of this measurement. The scaling is the same as for

the other xIRMS and xIRMSOS registers (see the Filter-Based

Total RMS section for more information).

The ADE9078 also calculates the rms of ISUM = IA + IB + IC ± IN

and stores the result in ISUMRMS. An ISUMRMSOS register

allows offset calibration of this measurement. The scaling is the

same as for the other xIRMS and xIRMSOS registers (see the

Filter-Based Total RMS section for more information).

If a neutral current sensor is not used, write 0 to the

ISUM_CFG bits in the CONFIG0 register, and then ISUMRMS

approximates the neutral current from the sum of IA, IB, and IC.

If the measured neutral current, NI_PCF, deviates from the sum

of AI_PCF + BI_PCF + CI_PCF current channel waveforms,

there may be a fault in the system.

To determine how big the mismatch is between the measured

neutral current and the measured Channel A, Channel B, and

Channel C currents, select ISUM_CFG[1:0] to 01 or 10 based

on the direction of the neutral current with respect to the other

current channel waveforms.

Table 14. ISUM Configuration Options

ISUM_CFG[1:0]

ISUM Calculation

00, 11

ISUM = AI_PCF + BI_PCF + CI_PCF

ISUM = AI_PCF + BI_PCF + CI_PCF + NI_PCF

ISUM = AI_PCF + BI_PCF + CI_PCF − NI_PCF

ISUMRMS has the same scaling as xIRMS. Note that if AI_PCF,

BI_PCF, and CI_PCF are all at full scale and in phase with each

other, with ISUM_CFG = 00 or 11, ISUMRMS is 3 × 52,866,837d =

158,600,511d. If AI_PCF, BI_PCF, CI_PCF, and NI_PCF are all

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