ADE7757 Analog Devices, ADE7757 Datasheet - Page 12

• Manufacturer Part Number: ADE7757
• Description: Energy Metering IC with Integrated Oscillator
• Manufacturer: Analog Devices

For the purpose of calibration, this integration time could
be 10 to 20 seconds in order to accumulate enough pulses
to ensure correct averaging of the frequency. In normal
operation the integration time could be reduced to one or
two seconds depending, for example, on the required up-
date rate of a display. With shorter integration times on
the MCU the amount of energy in each update may still
have some small amount of ripple, even under steady load
conditions. However, over a minute or more the measured
energy will have no ripple.
Calculating and displaying power information will always
have some associated ripple that will depend on the inte-
gration period used in the MCU to determine average
power and also the load. For example, at light loads the
output frequency may be 10 Hz. With an integration pe-
riod of two seconds, only about 20 pulses will be counted.
The possibility of missing one pulse always exists as the
ADE7757 output frequency is running asynchronously to
the MCU timer. This would result in a one-in-twenty or
5% error in the power measurement.
The ADE7757 calculates the product of two voltage signals (on
Channel V1 and Channel V2) and then low-pass filters this
product to extract real power information. This real power
information is then converted to a frequency. The frequency
information is output on F1 and F2 in the form of active low
pulses. The pulse rate at these outputs is relatively low,
e.g., 0.175 Hz maximum for ac signals with S0 = S1 =
0—see Table II. This means that the frequency at these
outputs is generated from real power information accumu-
lated over a relatively long period of time. The result is an
output frequency that is proportional to the average real
power. The averaging of the real power signal is implicit
to the digital-to-frequency conversion. The output fre-
quency or pulse rate is related to the input voltage signals
by the following equation:
where:
V1
V 2
V
Freq
F
ref
1
rms
rms
4
= Output frequency on F1 and F2 (Hz)
= Differential rms voltage signal on Channel V1
= Differential rms voltage signal on Channel V2
= The reference voltage (2.5 V ± 8%) (volts)
= One of four possible frequencies selected by us-
(volts)
(volts)
ing the logic inputs S0 and S1—see Table I.
Freq
515
.
84
V
1
rms
V
PRELIMINARY TECHNICAL DATA
ref
2
V
2
rms
F
1
4
–12–
NOTE
*F
In this example, with ac voltages of ±30 mV peak applied
to V1 and ±165 mV peak applied to V2, the expected
output frequency is calculated as follows:
The pulse output CF (Calibration Frequency) is intended for
calibration purposes. The output pulse rate on CF can be up to
2048 times the pulse rate on F1 and F2. The lower the F
frequency selected, the higher the CF scaling (except for the
high frequency mode SCF = 0, S1 = S0 = 1). Table III shows
how the two frequencies are related, depending on the states of
the logic inputs S0, S1 and SCF. Due to its relatively high
pulse rate, the frequency at CF logic output is proportional to
the instantaneous real power. As with F1 and F2, CF is derived
from the output of the low-pass filter after multiplication. How-
ever, because the output frequency is high, this real power
information is accumulated over a much shorter time. Hence
less averaging is carried out in the digital-to-frequency con-
version. With much less averaging of the real power signal, the
CF output is much more responsive to power fluctua-
tions—see Signal Processing Block in Figure 11.
V1
V 2
V
F
1
ref
1–4
rms
rms
4
is a binary fraction of the internal oscillator frequency
Freq
= 0.85 Hz, S0 = S1 = 0
= 0.03
= 0.165/
= 2.5 V (nominal reference value).
0
0
1
1
0
0
1
1
NOTE: If the on-chip reference is used, actual
output frequencies may vary from device to device
due to reference tolerance of ±8%.
515
2
.
2
volts
85
volts
0
1
0
1
2
0
1
0
1
0
.
03
2
0
.
2
165
.
5
2
0
.
85
0.85
1.7
3.4
6.8
0
0.175
0.35
0.7
1.4
.
REV. PrC.
175
1–4