ADE7858 datasheet(31/76 Pages) AD

31 / 76 page

Preliminary Technical Data

ADE7858

Rev. PrA | Page 31 of 76

- if ADE7858 registers located in the data memory RAM have

not been modified, write 0x0001 into register RUN[15:0] to

start the DSP.

-if ADE7858 registers located in the data memory RAM have to

be modified, first execute a software or a hardware reset,

initialize all ADE7858 registers at desired values and then write

0x0001 into register RUN[15:0] to start the DSP.

As mentioned in Power Management chapter, when the

ADE7858 switches out of PSM0 power mode, it is

recommended to stop the DSP by writing 0x0000 into

RUN[15:0] register (see Table 9 for recommended actions when

changing power modes).

ROOT MEAN SQUARE MEASUREMENT

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

an ac signal. Its definition can be both practical and mathemati-

cal. 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





(9)

For time sampling signals, rms calculation involves squaring the

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

]

[

2 n

FRMS





(10)

The expression (10) implies that for signals containing

harmonics, the rms calculation contains the contribution of all

harmonics, not only the fundamental. The method is to low-

pass filter the square of the input signal (LPF) and take the

square root of the result (see Figure 36).





 









sin

)

(

(11)

Then

 

































sin

)

cos(

)

(













(12)

After the LPF and the execution of the square root, the rms

value of f(t) is obtained:







(13)

The rms calculation based on this method is simultaneously

processed on all seven analog input channels. Each result is

available in 24-bit registers AIRMS, BIRMS, CIRMS, AVRMS,

BVRMS and CVRMS.

Current RMS Calculation

This chapter presents the first approach to compute the rms

values of all phase currents.

Figure 36 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. The current rms values are signed

24-bit values and they are stored into AIRMS[23:0],

BIRMS[23:0] and CIRMS[23:0]. The update rate of the current

rms measurement is 8KHz.

xIRMSOS[23:0]

LPF



xIRMS[23:0]

CURRENT SIGNAL

FROM HPF OR

INTEGRATOR

(IF ENABLED)

0x5A7540=

5,928,256

0xA58AC0=

-5,928,256

Figure 36. 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

±5,928,256. The equivalent rms value of a full-scale sinusoidal

signal is 4,191,910(0x3FF6A6), independent of the line

frequency. If the integrator is enabled, that is when bit 0

(INTEN) in CONFIG[15:0] register is set to 1, the equivalent

rms value of a full-scale sinusoidal signal at 50Hz is

4,191,910(0x3FF6A6) and at 60Hz is 3,493,258(0x354D8A).

The accuracy of the current rms is typically 0.1% error from the

full-scale input down to 1/1000 of the full-scale input. Additionally,

this measurement has a bandwidth of 2 kHz. It is recommended

to read the rms registers synchronous to the voltage zero

crossings to ensure stability. The

IRQ interrupt can be used to

indicate when a zero crossing has occurred (see the Interrupts

section).

Table 10 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 10. Settling Time for IRMS Measurement

50Hz Input signals

60Hz Input signals

Integrator Off

530msec

Integrator On

550msec

500msec

As previously stated, the serial ports of the ADE7858 work on

32, 16 or 8-bit words. Similar to the register presented in Figure

ADE7858 Datasheet(PDF) 31 Page - Analog Devices

ADE7858 Datasheet(HTML) 31 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76

Datasheet Download

Link URL