ADE7752B Datasheet, PDF (22/27 Pages) Analog Devices – Polyphase Energy Metering IC with Pulsed Output

ADE7752B
TRANSFER FUNCTION
FREQUENCY OUTPUTS F1 AND F2
The ADE7752B calculates the product of six voltage signals (on
current channel and voltage channel) and then low-pass filters
this product to extract active power information. This active
power information is then converted to a frequency. The
frequency information is output on F1 and F2 in the form of
active high pulses. The pulse rate at these outputs is relatively
low, for example, 2.01 Hz maximum for ac signals with SCF =
S0 = 0; S1 = 1 (see Table 6). This means that the frequency at
these outputs is generated from active power information
accumulated over a relatively long period. The result is an
output frequency that is proportional to the average active
power. The averaging of the active power signal is implicit to
the digital-to-frequency conversion. The output frequency or
pulse rate is related to the input voltage signals by the following
equation:
Freq = ^{6.181× (VAN × IA + VBN × IB + VCN × IC ) × F 1−7}
V_REF2
(11)
where:
Freq = output frequency on F1 and F2 (Hz).
VAN, VBN, and VCN = differential rms voltage signal on voltage
channels (V).
IA, IB, and IC = differential rms voltage signal on current channels
(V).
VREF = the reference voltage (2.4 V ±8%) (V).
F1–7 = one of seven possible frequencies selected by using the
logic inputs SCF, S0, and S1 (see Table 5).
Table 5. F1–7 Frequency Selection¹
SCF
S1
S0
0
0
0
1
0
0
0
0
1
1
0
1
0
1
0
1
1
0
0
1
1
1
1
1
F1–7 (Hz)
2.30
4.61
1.15
4.61
5.22
1.15
0.58
0.58
1 _{F1–7 is a fraction of the master clock and therefore varies if the specified}
CLKIN frequency is altered.
Preliminary Technical Data
Example 1
Thus, if full-scale differential dc voltages of +500 mV are
applied to VA, VB, VC, IA, IB, and IC, respectively (500 mV is
the maximum differential voltage that can be connected to
current and voltage channels), then the expected output
frequency is calculated as follows:
F1–7 = 0.58 Hz, SCF = S0 = S1 = 1
VAN = VBN = VCN = IA = IB = IC
= 500 mV dc = 0.5 V(rms of dc = dc)
VREF = 2.4 V (nominal reference value)
Note that if the on-chip reference is used, actual output
frequencies can vary from device to device due to reference
tolerance of ±8%.
Freq = 3 × ^{6.181× 0.5 × 0.5 × 0.58} = 0.467 Hz
2.4²
(12)
Example 2
In this example, with ac voltages of ±500 mV peak applied to
the voltage channels and current channels, the expected output
frequency is calculated as follows:
F1−7 = 0.58 Hz, SCF = S0 = S1 = 1
V_AN = V_BN = V_CN = IA = IB = IC
(13)
= 500 mV peak AC = ^0.5 Vrms
2
V_REF = 2.4 V (nominal reference value)
Note that if the on-chip reference is used, actual output fre-
quencies can vary from device to device due to reference
tolerance of ±8%.
Freq
= 3×
6.181× 0.5 × 0.5 × 0.58
2 × 2 × 2.4²
=
0.233
Hz
(14)
As can be seen from these two example calculations, the maximum
output frequency for ac inputs is always half of that for dc input
signals. The maximum frequency also depends on the number of
phases connected to the ADE7752B. In a 3-phase 3-wire delta ser-
vice, the maximum output frequency is different from the maxi-
mum output frequency in a 3-phase 4-wire Wye service. The
reason is that there are only two phases connected to the analog
inputs, but also that in a delta service, the current channel input
and voltage channel input of the same phase are not in phase in
normal operation.
Rev. PrA | Page 22 of 27