ADE7761ARS-REF View Datasheet(PDF) - Analog Devices
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ADE7761ARS-REF Datasheet PDF : 28 Pages
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ADE7761
voltage, therefore, is 0 V. The CT turns ratio and burden resistor
(RB) are selected to give a peak differential voltage of ±660 mV.
Figure 14 shows two typical connections for Channel V2. The
first option uses a potential transformer (PT) to provide
complete isolation from the main voltage. In the second option,
the ADE7761 is biased around the neutral wire, and a resistor
divider is used to provide a voltage signal that is proportional to
the line voltage. Adjusting the ratio of RA and RB + VR is a
convenient way of carrying out a gain calibration on the meter.
INTERNAL OSCILLATOR
The nominal internal oscillator frequency is 450 kHz when
used with the recommended ROSC resistor value of 6.2 kΩ
between RCLKIN and DGND (see Figure 16).
The internal oscillator frequency is inversely proportional to the
value of this resistor. Although the internal oscillator operates
when used with a ROSC resistor value between 5 kΩ and 12 kΩ, it
is recommended to choose a value within the range of the
nominal value.
Figure 15 shows a typical connection for MISCAL input. The
The output frequencies on CF, F1, and F2 are directly propor-
voltage reference input (REFIN/OUT) is used as a dc reference to
set the MISCAL voltage. Adjusting the level of MISCAL to
calibrate the meter in missing neutral mode can be done by
changing the ratio of RC and RD + VR1. When the internal
reference is used, the values of RC, RD, and VR1 must be chosen
E to limit the current sourced by the internal reference sourcing
current to below the specified 20 µA. Therefore, because VREF
internal = 2.5 V, RC + RD + VR1 > 600 kΩ.
T RF
V1A
CT
RB
E IP IN AGND
RB
CF
V1N
CF
L CT
RF
V1B
Figure 13. Typical Connection for Channel 1
O RF
V2P
±660mV CF
RF
V2N
CF
S AGND
RA*
RB*
B VR*
O *RB + VR = RF
CF
V2P
RF
V2N
CT
tional to the internal oscillator frequency; therefore, the resistor
ROSC must have a low tolerance and low temperature drift. A low
tolerance resistor limits the variation of the internal oscillator
frequency. Small variation of the clock frequency and conse-
quently of the output frequencies from meter to meter
contributes to a smaller calibration range of the meter. A low
temperature drift resistor directly limits the variation of the
internal clock frequency over temperature. The stability of the
meter to external variation is then better ensured by design.
ADE7761
2.5V
4kΩ
REFERENCE
INTERNAL
OSCILLATOR
9
REFIN/OUT
14
RCLKIN
ROSC
17
DGND
Figure 16. Internal Oscillator Connection
ANALOG-TO-DIGITAL CONVERSION
The analog-to-digital conversion in the ADE7761 is carried out
using second-order Σ-Δ ADCs. Figure 17 shows a first-order
(for simplicity) Σ-Δ ADC. The converter is made up of two
parts: the Σ-Δ modulator and the digital low-pass filter.
ANALOG
LOW-PASS FILTER
R
MCLK
INTEGRATOR
∫
LATCHED
DIGITAL
COMPAR- LOW-PASS FILTER
ATOR
VREF
1
24
Figure 14. Typical Connection for Channel 2
C
REFIN/OUT RC
RD
VR1
CF
MISCAL
RF
V2N
CF
Figure 15. Typical Connection for MISCAL
....10100101....
1-BIT DAC
Figure 17. First-Order Σ-∆ ADC
A Σ-Δ modulator converts the input signal into a continuous
serial stream of 1s and 0s at a rate determined by the sampling
clock. In the ADE7761, the sampling clock is equal to CLKIN.
The 1-bit DAC in the feedback loop is driven by the serial data
stream. The DAC output is subtracted from the input signal. If
the loop gain is high enough, the average value of the DAC
Rev. A | Page 14 of 28
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