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EVAL-AD7751/AD7755EB
AD7751 EVALUATION BOARD SET UP AS AN ENERGY
METER
Figure 8 shows a wiring diagram that allows a simple energy
meter to be implemented using the AD7751 evaluation board.
Because the AD7751 monitors both the phase and neutral cur-
rents, isolation is required on at least one of the current trans-
ducers. One convenient way to provide isolation and eliminate
problems with matching is to use two CTs (current transform-
ers). The CTs are connected as shown in Figure 8. The CTs
have a turns ratio of 1:2500. The burden resistance for the CTs
can be placed on the evaluation board at SH1 and SH2. The
meter is intended to be used with a line voltage of 240 V and a
maximum current of 60 A. The frequency outputs F1 and F2
can be used to drive a mechanical counter. These outputs will
be calibrated to provide 100 imp/kWhr. The logic output CF
has an output frequency that can be up to 128 times higher than
the frequency on F1 and F2. This output can be used for cali-
bration purposes and is shown connected to a frequency counter
via the optoisolator in Figure 8.
At maximum current (60 A) the power see by the meter will
be 14.4 kW. This will produce a frequency of 0.4 Hz on the
logic outputs F1 and F2 when these outputs are calibrated to
100imp/kWhr (100imp/hr = 0.02777 Hz, 0.02777 × 14.4 =
0.4 Hz). From Table III in the AD7751 data sheet, the clos-
est frequency to 0.4 Hz in the half-scale ac inputs column is for
F 3 , i.e., 0.34 Hz. Therefore F 3 is selected by setting S0 = 0 and
S1 = 1. The choice of CF frequencies in this mode (see Table
IV in the AD7751 data sheet) are 16 times F1 and 32 times F1.
For this example 32 times F1 is selected by setting SCF = 1.
Since the voltage on Channel 1 is fixed, the only possible way of
calibrating (adjusting) the output frequency on F1 and F2 is by
varying the voltage on Channel 2. This is carried out by varying
the attenuation of the line voltage using the trim pot.
First we can calculate the voltage required on Channel 2 in
order to calibrate the frequency on F1 and F2 to 100imp/kWhr.
The AD7751 data sheet gives the equation which relates the
voltage on Channel 1 and Channel 2 to the output frequency on
F1 and F2.
5.000V
SK3B
SK3A
5.000V
Freq =
5 . 74 × V 1 × V 2 × Gain × F 1 ? 4
2
V REF
(2)
NEUTRAL PHASE
AGND AVDD
JP20 = CLOSED
JP21 = CLOSED
First a current is selected for calibration, 15 A for example. This
gives a Channel 1 voltage of 15 A/2500 × 1.2 ? = 7.2 mV rms.
240V
1:2500
CT
SK2
V1A
SK5A
The gain of Channel 1 on the AD7755 is set to 8 (G0 = 0, G1 =
1). The on-chip or external reference of 2.5 V is selected using
JP4 = CLOSED
15A
15A
SH1
1.2
SH1
1.2
7.2mV
JP1 = OPEN
JP2 = OPEN
JP3 = OPEN
V1N
7.2mV JP5 = OPEN
JP6 = OPEN
3200
imp/kWhr
SK5B
SK5C
JP13.
The output frequency at 15 A on F1 and F2 should be
0.02777 Hz (100imp/kWhr) × 3.6 (240 V × 15 A = 3.6 kW) =
0.1 Hz.
JP7
JP15 = A
JP52
JP18 = B
CT
1:2500
TP11
AGND
SK1A
V1B JP12 = B
JP19A = CLOSED JP14 = A
JP22 = OPEN
JP16 = B
R53 JP17 = A
580k
From Equation 2 the voltage on Channel 2 should be set to
278 mV. The attenuation network as shown in Figure 1 is used
to attenuate 240 V to 278 mV. R53 = 580 k ? , R54 = 12 k ? ,
R56 = 500 ? and the trim pot R31 = 500 ? .
However since the meter is being calibrated at CF and CF is set
SK1B
LOAD
JP7 = OPEN
JP8 = OPEN
JP9 = OPEN
JP10 = CLOSED
R54
12k
JP8
C53
JP50 = P
JP51A = CLOSED
JP52 = CLOSED
R55
R31
500
R56
500
A
B
JP51
278mV
V2P
3.2000 Hz
to 32 times F1, the voltage on Channel 2 should be adjusted
until CF = 32 × 0.1 Hz = 3.2 Hz is registered on the frequency
counter. The counter should be set up to display the average of
ten frequency measurements on CF. This will remove any ripple
due to the instantaneous power signal. See the AD7751 data
sheet for more details.
JP11 = N
R55 = REMOVED
Figure 8. AD7751 Evaluation Board as an Energy Meter
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