ADE7758ARWRL datasheet(28/68 Pages) AD

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ADE7758

Rev. A | Page 28 of 68

REFERENCE CIRCUIT

The nominal reference voltage at the REFIN/OUT pin is 2.42 V.

This is the reference voltage used for the ADCs in the ADE7758.

However, the current channels have three input range selections

(full scale is selectable among 0.5 V, 0.25 V, and 0.125 V). This is

achieved by dividing the reference internally by 1, ½, and ¼. The

reference value is used for the ADC in the current channels. Note

that the full-scale selection is only available for the current inputs.

The REFIN/OUT pin can be overdriven by an external source, for

example, an external 2.5 V reference. Note that the nominal

reference value supplied to the ADCs is now 2.5 V and not 2.42 V.

This has the effect of increasing the nominal analog input signal

range by 2.5/2.42 × 100% = 3% or from 0.5 V to 0.5165 V.

The voltage of the ADE7758 reference drifts slightly with

temperature—see the Specifications section for the temperature

coefficient specification (in ppm/°C). The value of the tempera-

ture drift varies from part to part. Because the reference is used

for all ADCs, any x% drift in the reference results in a 2x%

deviation of the meter accuracy. The reference drift resulting

from temperature changes is usually very small and typically

much smaller than the drift of other components on a meter.

Alternatively, the meter can be calibrated at multiple

temperatures.

TEMPERATURE MEASUREMENT

The ADE7758 also includes an on-chip temperature sensor. A

temperature measurement is made every 4/CLKIN seconds.

The output from the temperature sensing circuit is connected to

an ADC for digitizing. The resultant code is processed and

placed in the temperature register (TEMP[7:0]). This register

can be read by the user and has an address of 0x11 (see the

ADE7758 Serial Interface section). The contents of the tempera-

ture register are signed (twos complement) with a resolution of

3°C/LSB. The offset of this register may vary from part to part

significantly. To calibrate this register, the nominal value should

be measured, and the equation should be adjusted accordingly.

For example, if the temperature register produces a code of 0x00

when the ambient temperature is approximately 70°C, the value

of the register is

Temperature Register = Temperature (°C) − 70

Depending on the nominal value of the register, some finite

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

compensated in the 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

1 ∫

(1)

For time sampling signals, rms calculation involves squaring the

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

[ ]

∑

2 n

FRMS

(2)

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 62).

With

i(t) =

sin(

× IRMS

then

i2(t) = IRMS2 − IRMS2× cos

(

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 band-limited to 160 Hz. The current rms,

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

Current RMS Calculation

Figure 62 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 unsigned 24-bit registers (AIRMS,

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

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