ADE7816 datasheet(23/48 Pages) AD

23 / 48 page

Data Sheet

ADE7816

Rev. 0 | Page 23 of 48

ROOT MEAN SQUARE MEASUREMENT

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

an ac signal. Specifically, the rms of an ac signal is equal to the

amount of dc required to produce an equivalent amount of power

in the load. The ADE7816 provides rms measurements on the

six current channels and the voltage channel simultaneously.

These measurements have a settling time of approximately 440 ms

with the integrator off and 500 ms with the integrator on. The

registers are updated every 125 μs. The rms value is measured

over a 2 kHz bandwidth.

The 24-bit, unsigned voltage rms measurement is available in the

VRMS register (Address 0x43C0). Similarly, the six current channel

rms measurements are available in the IARMS (Address 0x43C1),

IBRMS (Address 0x43C2), ICRMS (Address 0x43C3), IDRMS

(Address 0x43C4), IERMS (0x43C5), and IFRMS (Address 0x43C6)

registers. All registers are updated at a rate of 8 kHz. Figure 32

shows the IxRMS signal path. A similar signal path is used on

the voltage channel to compute the VRMS measurement.

Due to nonidealities in the internal filtering, it is recommended

that the IxRMS registers be read synchronously to the zero-

crossing signal (see the Zero-Crossing Detection section). This

helps to stabilize reading-to-reading variation by removing the

effect of any 2ω ripple that is present on the rms measurement.

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

rms value of a sinusoidal signal is 4,191,910 (0x3FF6A6),

independent of line frequency. If the integrator is enabled on

the current channels, the equivalent current rms value of a full-

scale sinusoidal signal at 50 Hz is 4,191,910 (0x3FF6A6). At

60 Hz, it is 3,493,258 (0x354D8A).

NO LOAD DETECTION

The ADE7816 includes a no load detection feature that eliminates

meter creep. Meter creep is defined as excess energy that is

accumulated by the meter when there is no load attached. The

ADE7816 warns of this condition and stops energy accumulation

if the energy falls below a programmable threshold. The ADE7816

includes a no load feature on the active and reactive energy

measurements. This allows a true no load condition to be

detected.

The no load condition is triggered when the absolute values of

the active and reactive powers are less than or equal to a thresh-

old that is specified in the APNOLOAD (Address 0x43AF) and

VARNOLOAD (Address 0x43B0) registers. When in the no

load condition, the active and reactive energies are no longer

accumulated in the energy registers. Note that each of the six

channels has a separate no load circuit.

Setting the No Load Thresholds

The APNOLOAD and VARNOLOAD registers are compared

to the active and reactive powers, respectively, to set the no load

threshold. With full-scale inputs on both the current and voltage

channel, the maximum power is 0x1FF6A6B. The no load

threshold should, therefore, be set with respect to this maxi-

mum power, as follows:

APNOLOAD =

(5)

0x1FF6A6B × V% of Full_Scale × I(noload)% of Full_Scale

For example, if the nominal voltage is set to 50% of full scale

and the current channel no load threshold is required to be at

0.01% of full scale, the APNOLOAD threshold is

APNOLOAD = 0x1FF6A6B × 50% × 0.01% = 0x68C

(6)

The VARNOLOAD register is usually set to the same value as

that of the APNOLOAD register. When the APNOLOAD and

VARNOLOAD registers are set to negative values, the no load

detection circuit is disabled.

Bit 0 (NLOAD1) in the STATUS1 register (Address 0xE503) is set

when the no load condition occurs on the A, B, or C current chan-

nel. Bit 1 (NLOAD2) in the STATUS1 register is set when the

load condition occurs on the D, E, or F current channel. Bits[5:0]

(NOLOADx) in the CHNOLOAD register (Address 0xE608)

can be used to determine which channel caused the no load

condition. When NOLOADx is cleared to 0, the channel is not in

a no load condition. When NOLOADx is set to 1, the channel is

in a no load condition.

No Load Interrupt

The ADE7816 includes two interrupts that are associated with

the no load feature. The first is associated with the A, B, and C

current channels, and it can be enabled by setting Bit 0 (NLOAD1)

in the MASK1 register (Address 0xE50B). The second interrupt

is associated with the D, E, and F current channels; it can be

enabled by setting Bit 1 (NLOAD2) in the MASK1 register. If

the corresponding interrupt is enabled, the no load condition

causes the external IRQ1 pin to go low (see the

section).

Interrupts

CURRENT SIGNAL FROM

HPF OR INTEGRATOR

(IF ENABLED)

LPF

√

IxRMSOS[23:0]

IxRMS[23:0]

Figure 32. IxRMS Signal Path

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