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Data Sheet

ADE7880

Rev. A | Page 41 of 104

Current RMS Offset Compensation

The ADE7880 incorporates a current rms offset compensation

NIRMSOS. These are 24-bit signed registers that are used to

remove offsets in the current rms calculations. An offset can

exist in the rms calculation due to input noises that are

integrated in the dc component of I2(t). One LSB of the current

rms offset compensation register is equivalent to one LSB of the

current rms register. Assuming that the maximum value from the

current rms calculation is 3,766,572 with full-scale ac inputs

(50 Hz), one LSB of the current rms offset represents 0.00045%

100

3767

128

3767

⎟

⎠

⎞

⎜

⎝

⎛

−

of the rms measurement at 60 dB down from full scale. Conduct

offset calibration at low current; avoid using currents equal to

zero for this purpose.

IRMSOS

rms

128

(15)

where I rms0 is the rms measurement without offset correction.

As stated in the Current Waveform Gain Registers section, the

serial ports of the ADE7880 work on 32-, 16-, or 8-bit words

and the DSP works on 28 bits. Similar to the register presented

in Figure 43, the AIRMSOS, BIRMSOS, CIRMSOS, and

NIRMSOS 24-bit signed registers are accessed as 32-bit registers

with four MSBs padded with 0s and sign extended to 28 bits.

Current Mean Absolute Value Calculation

This section presents the second approach to estimate the rms

values of all phase currents using the mean absolute value (mav)

method. This approach is used in PSM1 mode, to allow energy

accumulation based on current rms values when the missing

neutral case demonstrates to be a tamper attack. This data path

is active also in PSM0 mode to allow for its gain calibration.

The gain is used in the external microprocessor during PSM1

mode. The mav value of the neutral current is not computed

using this method. Figure 66 shows the details of the signal

processing chain for the mav calculation on one of the phases of

the current channel.

CURRENT SIGNAL

COMING FROM ADC

xIMAV[23:0]

HPF

|X|

Figure 66. Current MAV Signal Processing for PSM1 Mode

The current channel mav value is processed from the samples

used in the current channel waveform sampling mode. The

samples are passed through a high-pass filter to eliminate the

eventual dc offsets introduced by the ADCs and the absolute

values are computed. The outputs of this block are then filtered

to obtain the average. The current mav values are unsigned 20-bit

values and they are stored in the AIMAV, BIMAV, and CIMAV

registers. The update rate of this mav measurement is 8 kHz.

207000

207500

208000

208500

209000

209500

210000

210500

211000

211500

212000

FREQUENCY (Hz)

Figure 67. xIMAV Register Values at Full Scale, 45 Hz to 65 Hz Line

Frequencies

The mav values of full-scale sinusoidal signals of 50 Hz and

60 Hz are 209,686 and 210,921, respectively. As seen in Figure 67,

there is a 1.25% variation between the mav estimate at 45 Hz

and the one at 65 Hz for full-scale sinusoidal inputs. The accuracy

of the current mav is typically 0.5% error from the full-scale

input down to 1/100 of the full-scale input. Additionally, this

measurement has a bandwidth of 3.3 kHz. The settling time for

the current mav measurement, that is the time it takes for the

mav register to reflect the value at the input to the current

channel within 0.5% error, is 500 ms.

As stated in the Current Waveform Gain Registers section, the

serial ports of the ADE7880 work on 32-, 16-, or 8-bit words.

As presented in Figure 68, the AIMAV, BIMAV, and CIMAV 20-

bit unsigned registers are accessed as 32-bit registers with the 12

MSBs padded with 0s.

20 19

20-BIT UNSIGNED NUMBER

0000 0000 0000

Figure 68. xIMAV Registers Transmitted as 32-Bit Registers

Current MAV Gain and Offset Compensation

The current rms values stored in the AIMAV, BIMAV, and

CIMAV registers can be calibrated using gain and offset

coefficients corresponding to each phase. It is recommended to

calculate the gains in PSM0 mode by supplying the ADE7880

with nominal currents. The offsets can be estimated by

supplying the ADE7880 with low currents, usually equal to the

minimum value at which the accuracy is required. Every time

the external microcontroller reads the AIMAV, BIMAV, and

CIMAV registers, it uses these coefficients stored in its memory

to correct them.

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