ADE75 datasheet(52/148 Pages) AD

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ADE75xx/ADE71xx

Preliminary Technical Data

Rev. PrE | Page 52 of 148

Figure 34. ADE75XX/ADE71XX RMS Signal Processing

The rms signals can be read from the waveform register by

setting the WAVMODE register (0x0D) and setting the WFSM

bit in the Interrupt Enable Register 3 SFR (MIRQENH, 0xDB).

Like the current and voltage channels waveform sampling

modes, the waveform date is available at sample rates of 27.9

kSPS, 14 kSPS, 7 kSPS, or 3.5 kSPS.

Important:

When the current input is larger than 40% of Full

scale, the Irms waveform sample register does not represent the

true rms value processed. The rms value processed with this

level of input is larger than the 24 bit read by the waveform

Current Channel RMS Calculation

The ADE75XX/ADE71XX simultaneously calculates the rms

values for the Current and Voltage channel in different registers.

The current channel rms calculation is done on the channel

selected by SEL_I_CH bits in the CALMODE register (0x3D).

Figure 35 shows the detail of the signal processing chain for the

rms calculation on the current channel. The current channel

rms value is processed from the samples used in the current

channel waveform sampling mode. The current channel rms

value is stored in an unsigned 24-bit register (IRMS). One LSB

of the current channel rms register is equivalent to one LSB of a

current channel waveform sample.

The update rate of the current channel rms measurement is

MCLK/5. To minimize noise in the reading of the register, the

Irms register can also be configured to be updated only with the

zero crossing of the voltage input. This configuration is done by

setting ZXRMS bit in the MODE2 register (0x0C).

With the specified full-scale analog input signal of 0.5 V, the

ADC produces an output code that is approximately

±0d2,684,354—see the Current Channel ADC section. The

equivalent rms value of a full-scale ac signal is 0d1,898,124

(0x1CF68C). The current rms measurement provided in the

ADE75XX/ADE71XX is accurate to within 0.5% for signal

input between full scale and full scale/1000. The conversion

from the register value to amps must be done externally in the

microprocessor using an amps/LSB constant.

IRMS(t)

LPF3

HPF1

0x00

IRMSOS[11:0]

IRMS[23:0]

226

225

sgn

227

217 216

218

DIGITAL

INTEGRATOR*

HPF

MODE1[5]

HPF

CURRENT CHANNEL

WAVEFORM

DATA RANGE WITH

INTEGRATOR OFF

0xD70A3E

0x000000

0x28F5C2

60Hz

CURRENT CHANNEL

WAVEFORM

DATA RANGE WITH

INTEGRATOR ON (60Hz)

0xD487B0

0x000000

0x2B7850

Figure 35. Current channel RMS Signal Processing

Current channel RMS Offset Compensation

The ADE75XX/ADE71XX incorporates a current channel rms

offset compensation register (IRMSOS). This is a 12-bit signed

rms calculation. An offset could exist in the rms calculation due

to input noises that are integrated in the dc component of V2(t).

The offset calibration allows the content of the IRMS register to

be maintained at 0 when no input is present on current channel.

One LSB of the current channel rms offset is equivalent to

16,384 LSB of the square of the current channel rms register.

Assuming that the maximum value from the current channel

rms calculation is 0d1,898,124 with full-scale ac inputs, then 1

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