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EVAL-AD7751/AD7755EB
AD7751/AD7755 OUTPUT FREQUENCY SELECTION
AD7751 and AD7755 provide up to four different output fre-
quencies on F1 and F2. The output frequency selection is made
via the logic inputs S0 and S1—see AD7751/AD7755 data
sheet. On the evaluation board these inputs are set by using
jumpers JP15 and JP16. The logic input SCF is set via jumper
14 (JP14). For a full explanation of the AD7751/AD7755 out-
put frequency selection see the data sheet.
AD7751/AD7755 INPUT GAIN SELECTION
AD7751 and AD7755 provide up to four different gain settings
on the analog input Channel 1. These gain settings are 1, 2, 8
and 16. The gain selection on the evaluation board is made via
JP17 and JP18.
EXTERNAL CLOCK INPUT
AD7751 and AD7755 are specified with a CLKIN value of
3.579545 MHz. The evaluation board uses a crystal of this value
for the on-chip gate oscillator circuit. However, an input is
provided to allow an external clock source to be used. An imped-
ance matching resistor (R11) of 50 ? is also available on the
board. NOTE: when using the on-chip oscillator this resistor
must be removed or the on-chip oscillator circuit will not start
up.
AD7755 EVALUATION BOARD SET UP AS AN ENERGY
METER
Figure 7 shows a wiring diagram that allows a simple energy
meter to be implemented using the AD7755 evaluation board.
The current transducer used in this example is a shunt (400 μ? ).
The meter is intended to be used with a line voltage of 220 V
and a maximum current of 25 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
calibration purposes and is shown connected to a frequency
counter via the optoisolator in Figure 7.
At maximum current (25 A), the power seen by the meter will
be 5.5 kW. This will produce a frequency of 0.153 Hz on F1
and F2 when these outputs are calibrated to 100imp/kWhr
(100imp/hr = 0.02777 Hz, 0.02777 × 5.5 = 0.153 Hz). From
Table III in the AD7755 data sheet, the closest frequency to
0.153 Hz in the half-scale ac inputs column is for F 2 , i.e., 0.17 Hz.
Therefore F 2 is selected by setting S0 = 1 and S1 = 0. The
choice of CF frequencies in this mode (see Table IV in the
AD7755 data sheet) are 32 times F1 and 64 times F1. For this
example 64 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 in F1 and F2 is by
5.000V
SK3B
SK3A
5.000V
JP20 = CLOSED
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 in Channel 2 in order
NEUTRAL PHASE
AGND AVDD
JP21 = CLOSED
to calibrate the frequency on the logic outputs F1 and F2 to
100imp/kWhr. The AD7755 data sheet gives the equation which
8 . 06 × V 1 × V 2 × Gain × F 1 ? 4
Freq = (1)
220V
SHUNT
5A
SK2B
2mV
SK2C
TP11
AGND
V1N
V1P
JP1 = OPEN
JP2 = CLOSED
JP3 = OPEN
JP4 = OPEN
JP5 = OPEN
JP6 = OPEN
JP12 = B
JP19A = CLOSED
JP22 = CLOSED
SK5A
6400
imp/kWhr
SK5B
SK5C
JP14 = A
JP15 = B
relates the voltage on Channel 1 and Channel 2 to the output
frequency on F1 and F2.
2
V REF
First a current is selected for calibration, 5 A for example. This
gives a Channel 1 voltage of 400 μ? × 5 A = 2 mV rms. The
gain of Channel 1 on the AD7755 is set to 16 (G0 = G1 = 1).
JP7
JP16 = A
JP17 = A
The on-chip or external reference of 2.5 V is selected using
SK1A
R53
JP52
JP18 = A
JP13.
SK1B
LOAD
JP7 = OPEN
JP8 = OPEN
JP9 = OPEN
R54
JP8
C53
JP50 = P
JP51A = CLOSED
R55
R31
R56
A
B
JP51
218mV
V2P
1.9555 Hz
The output frequency at 5 A on F1 and F2 should be 0.02777 Hz
(100imp/kWhr) × 1.1 (220 V × 5 A = 1.1 kW) = 0.03055 Hz.
From Equation 1 the voltage on Channel 2 should be set to
218 mV. The attenuation network as shown in Figure 1 is used
to attenuate 220 V to 218 mV. R53 = 660 k ? , R54 = 100 k ? ,
R56 = 500 ? and the trim pot R31 = 500 ? .
JP10 = CLOSED
JP11 = N
JP52 = CLOSED
R55 = REMOVED
However, since the meter is being calibrated at CF and CF is
Figure 7. AD7755 Evaluation Board as an Energy Meter
–4 –
set to 64 times F1, the voltage on Channel 2 should be adjusted
until CF = 64 × 0.03055 Hz = 1.9555 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 AD7755 data sheet for more details.
REV. 0
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