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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） |
| 文件頁數(shù)： | 12/16頁 |
| 文件大?。?/td> | 155K |
| 代理商： | AD7750 |
AD7750
–12–
REV. 0
1
See IEC 1036 2nd Edition 1996-09 Section 3.5.1.1.
T he high frequency F
OUT
output is intended to be used for
communications (via IR LED) and calibration purposes. F
OUT
produces a 90 ms wide pulse at a frequency that is proportional
to the product of Channel 1 and Channel 2 or the instanta-
neous voltage on Channel 1 or Channel 2. T he output fre-
quencies are given in T able I in the Determining the Output
Frequencies of the AD7750 section of this data sheet. As in
the case of F1 and F2, if the period of F
OUT
falls below 180 ms,
the F
OUT
pulsewidth is set to half the period. For example, if the
F
OUT
frequency is 20 Hz, the F
OUT
pulsewidth is 25 ms.
F1
F2
F
OUT
t
1
V
DD
0V
V
DD
V
DD
0V
0V
t
6
t
2
t
3
t
4
t
5
Figure 18. Timing Diagram for Frequency Outputs
VOLT AGE RE FE RE NCE
T he AD7750 has an on-chip temperature compensated band-
gap voltage reference of 2.5 V with a tolerance of
±
250 mV.
T he temperature drift for the reference is specified at 50 ppm/
°
C.
It should be noted that this reference variation will cause a
frequency output variation from device to device for a given set
of input signals. T his should not be a problem in most applica-
tions since it is a straight gain error that can easily be removed
at the calibration stage.
RE VE RSE POLARIT Y INDICAT OR
When the AD7750 is operated in a Magnitude Only mode of
operation (i.e., Modes 1, 2, 5 and 6), and the polarity of the
power changes, the logic output REVP will go high. However,
the REVP pin is only activated when the there is pulse output
on F1 or F2. T herefore, if the power being measured is low, it may
be some time before the REVP pin goes logic high even though the
polarity of the power is reversed. Once activated the REVP output
will remain high until the AD7750 is powered down.
APPLICAT IONS INFORMAT ION
D esigning a Single Phase Class 1 E nergy Meter (IE C 1036)
T he AD7750 Product-to-Frequency Converter is designed for
use in a wide range of power metering applications. In a typical
power meter two parameters are measured (i.e., line voltage and
current) and their product obtained. T he real power is then
obtained by low-pass filtering this product result. T he line
voltage can be measured through a resistor divider or voltage
transformer, and the current can be sensed and converted to
a voltage through a shunt resistor, current transformer or hall
effect device.
T he design methodology used in the following example is to use
the upper end of the current channel dynamic range, i.e., Chan-
nel 1 of the AD7750. T he assumption here is that the signal on
the voltage channel will remain relatively constant while the
signal on the current channel will vary with load. Using the
upper end of the dynamic range of Channel 1 will improve the
meter accuracy with small load currents. Hence an error of less
than 1% from 4% Ib to 400% Ib will be easier to achieve.
We will assume the design of a Class 1 meter. T he specification
(IEC1036) requires that the meter have an error of no greater
than 1% over the range 4% Ib to 400% Ib (I
MAX
), where Ib is
the basic current
1
. In addition, we will design a meter that ac-
commodates signals with a crest factor of 2. T he crest factor is
the ratio of V
PEAK
/V rms. A pure sinusoidal waveform has a crest
of sqrt(2) = 1.414 and an undistorted triangular waveform has a
crest factor of sqrt(3) = 1.73. Using a gain of 1 on Channel 1
the maximum differential signal which can be applied to Chan-
nel 1 is
±
2 V—See Analog Input Ranges section. With a crest
factor of 2 the maximum rms signal on Channel 1 is, therefore,
1 V rms (equivalent to I
MAX
). T he smallest signal (4% Ib) ap-
pearing on Channel 1 is therefore 10 mV rms.
Load Current
Channel 1
4% Ib
10 mV rms
Ib
250 mV rms
400 Ib
1 V rms
2
0. 2
0. 02
0. 002
1
0.01
400% Ib
4% Ib
C
Figure 19. Use the Upper End of the Dynamic Range of
Channel 1 (Current)
Calculations for a 100 PPK WHR Meter
T he AD7750 offers a range of maximum output frequencies—
see T able I and T able II. In the Magnitude Only modes of
operation the two maximum output frequencies are 1.45 Hz
and 2.9 Hz. T he signal on the voltage channel (Channel 2) is
scaled to achieve the correct output pulse frequency for a given
load (e.g., 100 PPK WHR). T he relationship between the input
signals and the output frequency is given by the equation:
Freq
=
k
×
F
MAX
where
k
= (1.32
×
V
1
×
V
2
×
Gain
)/
V
REF
F
MAX
= 6.8 Hz or 13.6 Hz depending on the mode—see T able
I,
Gain
is the gain of Channel 1,
V
1
and
V
2
are the differential
voltages on Channels 1 and 2 and
V
REF
is the reference voltage
(2.5 V
±
8%).
T o design a 100 PPK WHR meter with Ib = 15 A rms and a line
voltage of 220 V rms the output pulse frequency with a load
current of Ib is 0.0916 Hz (See Calculation 1 below).
T herefore, 0.0916 Hz = k
×
6.8 Hz (Mode 2) or k = 0.01347.
With a load current of Ib the signal on Channel 1 (V
1
) is equal
to 0.25 V rms (remember 400% Ib = 1 V rms) and, therefore,
the signal on Channel 2 (V
2
) is equal to 0.255 V rms (See Calcula-
tion 2). T his means that the nominal line voltage (220 V rms)
needs to be attenuated by approximately 860, i.e., 220/0.255.
2
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