ad7755 Analog Devices, Inc., ad7755 Datasheet - Page 11

• Manufacturer Part Number: ad7755
• Description: Energy Metering Ic With Pulse Output
• Manufacturer: Analog Devices, Inc.

Using Equations 1 and 2, the real power P can be expressed in
terms of its fundamental real power (P
power (P
where:
and
As can be seen from Equation 4 above, a harmonic real power
component is generated for every harmonic, provided that har-
monic is present in both the voltage and current waveforms.
The power factor calculation has previously been shown to be
accurate in the case of a pure sinusoid, therefore the harmonic
real power must also correctly account for power factor since it
is made up of a series of pure sinusoids.
Note that the input bandwidth of the analog inputs is 14 kHz
with a master clock frequency of 3.5795 MHz.
ANALOG INPUTS
Channel V1 (Current Channel )
The voltage output from the current transducer is connected to
the AD7755 here. Channel V1 is a fully differential voltage
input. V1P is the positive input with respect to V1N.
The maximum peak differential signal on Channel 1 should be
less than ± 470 mV (330 mV rms for a pure sinusoidal signal) for
specified operation. Note that Channel 1 has a programmable
gain amplifier (PGA) with user selectable gain of 1, 2, 8 or 16
(see Table I). These gains facilitate easy transducer interfacing.
The diagram in Figure 22 illustrates the maximum signal levels
on V1P and V1N. The maximum differential voltage is ±470 mV
divided by the gain selection. The differential voltage signal on
the inputs must be referenced to a common mode, e.g. AGND.
The maximum common mode signal is ± 100 mV as shown in
Figure 22.
+470mV
–470mV
φ
φ
P
P
1
1
h
H
=
=
V
=
=
CM
H
V
α
).
V1
α
∑
h
1
1
∝
h
≠
×
1
–
Vh
–
I
β
1
β
1
cos
h
×
Ih
φ
1
cos
DIFFERENTIAL INPUT
P
470mV MAX PEAK
φ
COMMON-MODE
h
=
P
100mV MAX
1
+
P
H
1
) and harmonic real
AGND
V1
V
CM
V1P
V1N
(3)
(4)
G1
0
0
1
1
Channel V2 (Voltage Channel )
The output of the line voltage transducer is connected to the
AD7755 at this analog input. Channel V2 is a fully differential
voltage input. The maximum peak differential signal on Chan-
nel 2 is ± 660 mV. Figure 23 illustrates the maximum signal
levels that can be connected to the AD7755 Channel 2.
Channel 2 must be driven from a common-mode voltage, i.e.,
the differential voltage signal on the input must be referenced to
a common mode (usually AGND). The analog inputs of the
AD7755 can be driven with common-mode voltages of up to
100 mV with respect to AGND. However best results are
achieved using a common mode equal to AGND.
Typical Connection Diagrams
Figure 24 shows a typical connection diagram for Channel V1.
A CT (current transformer) is the current transducer selected for
this example. Notice the common-mode voltage for Channel 1
is AGND and is derived by center tapping the burden resistor
to AGND. This provides the complementary analog input sig-
nals for V1P and V1N. The CT turns ratio and burden resistor
Rb are selected to give a peak differential voltage of ± 470 mV/
Gain at maximum load.
IP
PHASE
+660mV
–660mV
V
CM
NEUTRAL
V2
G0
0
1
0
1
Table I. Gain Selection for Channel 1
AGND
CT
DIFFERENTIAL INPUT
Gain
1
2
8
16
660mV MAX PEAK
Rb
COMMON-MODE
100mV MAX
Rf
Rf
470mV
GAIN
Maximum
Differential Signal
± 470 mV
± 235 mV
± 60 mV
± 30 mV
AGND
V2
V
CM
Cf
Cf
V2P
AD7755
V2N
V1P
V1N