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參數(shù)資料
| 型號(hào)： | AD7750 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Product-to-Frequency Converter(乘積-頻率轉(zhuǎn)換器) |
| 中文描述： | 產(chǎn)品頻率轉(zhuǎn)換器（乘積-頻率轉(zhuǎn)換器） |
| 文件頁(yè)數(shù)： | 13/16頁(yè) |
| 文件大?。?/td> | 155K |
| 代理商： | AD7750 |
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AD7750
–13–
REV. 0
For 100 PPK WHR V
2
is equal to 0.255 V rms or the line volt-
age attenuated by a factor of 860.
Calculation 1
100 PPK WHR = 0.02777 Hz/kW.
Ib of 15 A rms and line voltage of 220 V = 3.3 k
. Hence, the
output frequency is given by 3.3
×
0.02777 Hz = 0.0916 Hz.
Calculation 2
k = (1.32
×
V
1
×
V
2
×
Gain)/V
REF
0.01347 = (1.32
×
0.25
×
V
2
×
1)/6.25.
V
2
= 0.255.
Figure 21 below shows how the design equations from the previ-
ous page are implemented.
2
.
Measuring the Load Current
T he load current is converted to a voltage signal for Channel 1
using a CT (Current T ransformer). A 15 A rms load should
produce a 250 mV rms signal on Channel 1. A CT with a turns
ratio of 120 and a shunt resistor of 2
. will carry out the neces-
sary current to voltage conversion. T he CT and its shunt resis-
tance should be placed as close as possible to the AD7750. T his
will improve the accuracy of the meter at very small load cur-
rents. At small load currents the voltage levels on Channel 1 are
in the order of 10 mV and the meter is more prone to error due
to stray signal “pick up.” When measuring power the HPF in
the current channel must be switched on. T his is done by con-
necting the ACDC pin to V
DD
.
NOT E: T he voltage signals on V
1+
and V
1–
must be referenced
to ground. T his can be achieved as shown in Figure 21 below,
i.e., by referencing 1/2 R
CT
to ground or by connecting a
centertap on the CT secondary to ground.
Measuring the Line Voltage
When the AD7750 is biased around the live wire as shown in
Figure 21, the task of measuring the line voltage is greatly
simplified. A resistor divider attenuates the line voltage and
provides a single-ended input for Channel 2. T he component
values of the divider are chosen to give the correct rating (e.g.,
100 PPK WHR) for the meter. See the design equations on the
previous page. For this design an attenuation ratio of 860:1 is
required.
Antialiasing Components Channels 1 and 2
T he AD7750 is basically two ADCs and a digital multiplier. As
with any ADC, a LPF (Low-Pass Filter) should be used on the
analog inputs to avoid out of band signal being aliased into the
band of interest. In the case of a Class 1 meter the band of
interest lies in the range 48 Hz to 1 kHz approximately. T he
components R3, R4, R6, R7, C5, C6, C9 and C10 make up the
LPFs on each of the four analog inputs. Note that although
Channel 2 is used single ended a LPF is still required on V
2–
.
Power Supply Circuit
T he AD7750 operates from a single power supply of 5 V
±
5%
but still accommodates input signals in the range
±
1 V. Because
the AD7750 doesn’t require dual supplies the number of exter-
nal components for the power supply is reduced. One of the
most important design goals for the power supply is to ensure
that the ripple on the output is as low as possible. Every analog
or mixed signal IC is to a greater or lesser extent susceptible to
power supply variations. Power supply variations or ripple, if
large enough, may affect the accuracy of the device when mea-
suring small signals. T he plot in Figure 20 shows the ripple
associated with the circuit in Figure 21. T he ripple is in the
region of 10 mV peak to peak.
TIME
2
ms
5.065
5.060
5.050
600
620
V
5.055
640
660
680
700
720
740
760
780
800
Figure 20. Power Supply Ripple
C1
CT
120:1
R1
*BIASING AROUND THE LIVE WIRE PATENTED BY SCHLUMBERGER.
XTAL 3.57954MHz
C2
R2
Z1
D1
R4
R3
C5
C6
1/2 R
CT
1
V
R6
V
DD
C9
C10
C12
C11
C8
C7
MOV
SOURCE
LOAD
D2
D3
D4
R7
R8
R9
14
13
12
11
17
16
15
20
19
18
10
9
8
1
2
3
4
7
6
5
TOP VIEW
(Not to Scale)
AD7750
M
MODE2
V
DD
V
DD
V
DD
*
1/2 R
CT
1
V
VR1
R5
PULSE COUNTER
IR DIODE FOR
CALIBRATION
REVERSE POLARITY
INDICATOR
D5
C4
C3
P
N
Figure 21. Suggested Class 1 Meter Implementation
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