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ADE7758
Temp (°C) =
Current RMS Calculation
Data Sheet
[( TEMP [7:0] ? Offset ) × 3°C/LSB] + Ambient (°C)
(4)
Figure 63 shows the detail of the signal processing chain for the
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).
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.
AIRMSOS[11:0]
The ADE7758 temperature register varies with power supply. It
is recommended to use the temperature register only in
0x2851EC
SGN
2 24
2 23 2 22
2 16 2 15 2 14
applications with a fixed, stable power supply. Typical error with
respect to power supply variation is show in Table 5.
0x0
0xD7AE14
0x1D3781
0x00
Table 5. Temperature Register Error with Power Supply
Variation
4.5 V 4.75 V 5V 5.25 V 5.5 V
CURRENT SIGNAL
FROM HPF OR
INTEGRATOR
(IF ENABLED)
X 2
LPF3
+
+
AIRMS[23:0]
Register Value
% Error
219
+2.34
216
+0.93
214
0
211
?1.40
208
?2.80
Figure 63. Current RMS Signal Processing
With the specified full-scale analog input signal of 0.5 V, the
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
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
FRMS =
1
T
∫ 0
T
f 2 ( t ) dt
(5)
crossings to ensure stability. The IRQ can be used to indicate
when a zero crossing has occurred (see the Interrupts section).
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root.
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.
∑ f 2 [ n ]
FRMS =
1
N
N
n = 1
(6)
Table 6. Settling Time for IRMS Measurement
63% 100%
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
Integrator Off
Integrator On
80 ms
40 ms
960 ms
1.68 sec
the square root of the result (see Figure 63).
then
i ( t ) = √2 × IRMS × sin( ωt )
i 2 ( t ) = IRMS 2 ? IRMS 2 × cos( ωt )
(7)
(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.
Rev. E | Page 28 of 72
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