ADE7758_15 datasheet(28/72 Pages) AD

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ADE7758

Data Sheet

Rev. E | Page 28 of 72

Temp (°C) =

[(TEMP[7:0] − Offset) × 3°C/LSB] + Ambient(°C)

(4)

For example, if the temperature register produces a code of 0x46

at ambient temperature (25°C), and the temperature register

currently reads 0x50, then the temperature is 55°C :

Temp (°C) = [(0x50 – 0x46) × 3°C/LSB] + 25°C = 55°C

Depending on the nominal value of the register, some finite

temperature can cause the register to roll over. This should be

compensated for in the system master (MCU).

The ADE7758 temperature register varies with power supply. It

is recommended to use the temperature register only in

applications with a fixed, stable power supply. Typical error with

respect to power supply variation is show in Table 5.

Table 5. Temperature Register Error with Power Supply

Variation

4.5 V

4.75 V

5 V

5.25 V

5.5 V

219

216

214

211

208

% Error

+2.34

+0.93

−1.40

−2.80

ROOT MEAN SQUARE MEASUREMENT

Root mean square (rms) is a fundamental measurement of the

magnitude of an ac signal. Its definition can be both practical

and mathematical. Defined practically, the rms value assigned

to an ac signal is the amount of dc required to produce an

equivalent amount of power in the load. Mathematically, the

rms value of a continuous signal f(t) is defined as

()dt

FRMS

∫

(5)

For time sampling signals, rms calculation involves squaring the

signal, taking the average, and obtaining the square root.

]

[

2 n

FRMS

∑

(6)

The method used to calculate the rms value in the ADE7758 is

to low-pass filter the square of the input signal (LPF3) and take

the square root of the result (see Figure 63).

i(t) = √2 × IRMS × sin(ωt)

(7)

then

i2(t) = IRMS2 − IRMS2 × cos(ωt)

(8)

The rms calculation is simultaneously processed on the six

analog input channels. Each result is available in separate

registers.

While the ADE7758 measures nonsinusoidal signals, it should

be noted that the voltage rms measurement, and therefore the

apparent energy, are bandlimited to 260 Hz. The current rms as

well as the active power have a bandwidth of 14 kHz.

Current RMS Calculation

Figure 63 shows the detail of the signal processing chain for the

rms calculation on one of the phases of the current channel.

The current channel rms value is processed from the samples

used in the current channel waveform sampling mode. The

current rms values are stored in 24-bit registers (AIRMS,

BIRMS, and CIRMS). One LSB of the current rms register is

equivalent to one LSB of the current waveform sample. The

update rate of the current rms measurement is CLKIN/12.

SGN 224 223 222

216 215 214

CURRENT SIGNAL

FROM HPF OR

INTEGRATOR

(IF ENABLED)

0x1D3781

0x00

0x2851EC

0x0

0xD7AE14

LPF3

AIRMS[23:0]

AIRMSOS[11:0]

Figure 63. Current RMS Signal Processing

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

ADC produces an output code that is approximately

±2,642,412d (see the Current Channel ADC section). The

equivalent rms value of a full-scale sinusoidal signal at 60 Hz is

1,914,753 (0x1D3781).

The accuracy of the current rms is typically 0.5% error from the

full-scale input down to 1/500 of the full-scale input. Additionally,

this measurement has a bandwidth of 14 kHz. It is recommended

to read the rms registers synchronous to the voltage zero

crossings to ensure stability. The IRQ can be used to indicate

when a zero crossing has occurred (see the Interrupts section).

Table 6 shows the settling time for the IRMS measurement,

which is the time it takes for the rms register to reflect the value

at the input to the current channel.

Table 6. Settling Time for IRMS Measurement

63%

100%

Integrator Off

80 ms

960 ms

Integrator On

40 ms

1.68 sec

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