ADE7758 Analog Devices, ADE7758 Datasheet - Page 28

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

ADE7758
REFERENCE CIRCUIT
The nominal reference voltage at the REF
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 REF
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
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.
Temperature Register = Temperature (°C) − 70
IN/OUT
pin can be overdriven by an external source, for
IN/OUT
pin is 2.42 V.
Rev. A | Page 28 of 68
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
Register Value
% Error
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
For time sampling signals, rms calculation involves squaring the
signal, taking the average, and obtaining the square root.
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
then
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
i(t) =
i
FRMS
FRMS
2
(t) = IRMS
=
=
2
× IRMS
T
1
N
1
2
− IRMS
∫
n
0
∑
N
T
=
4.5 V
219
2.34
1
f
f
×
2
2
( )
sin(
[ ]
t
n
2
dt
× cos
ω
4.75 V
216
0.93
t)
(ω
t)
5 V
214
0
5.25 V
211
−1.40
5.5 V
208
−2.80
(1)
(2)