Datasheet

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
Register Value
219 216 214 211 208
% Error
+2.34 +0.93 0 −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
T
1
2
0
T
tfFRMS
∫
=
(5)
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root.
][
1
1
2
nf
N
FRMS
N
n
∑
=
=
(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
i
2
(t) = IRMS
2
− IRMS
2
× 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 2
24
2
23
2
22
2
16
2
15
2
14
CURRENT SIGNAL
FROM HPF OR
INTEGRATOR
(IF ENABLED)
0x1D3781
0x00
+
+
0x2851EC
0x0
0xD7AE14
X
2
LPF3
AIRMS[23:0]
AIRMSOS[11:0]
04443-062
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