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.
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).
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.
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
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
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.
The output frequencies on CF, F1, and F2 are directly propor-
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.
V
R
F
1A
CT
ADE7761
RB
RB
C
C
F
F
IP
IN
AGND
4k
Ω
2.5V
INTERNAL
OSCILLATOR
V
V
1N
1B
REFERENCE
9
14
17
CT
REF
RCLKIN
DGND
IN/OUT
R
F
R
OSC
Figure 13. Typical Connection for Channel 1
Figure 16. Internal Oscillator Connection
V
R
R
2P
F
C
C
±660mV
AGND
F
V
ANALOG-TO-DIGITAL CONVERSION
2N
F
F
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.
RA*
RB*
VR*
C
F
V
2P
MCLK
ANALOG
LOW-PASS FILTER
V
R
2N
F
LATCHED
COMPAR-
ATOR
INTEGRATOR
DIGITAL
LOW-PASS FILTER
C
T
R
*RB + VR = RF
∫
V
REF
1
24
C
Figure 14. Typical Connection for Channel 2
....10100101....
REF
IN/OUT
1-BIT DAC
RC
C
F
RD
Figure 17. First-Order Σ-∆ ADC
MISCAL
VR1
V
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
R
2N
F
C
F
Figure 15. Typical Connection for MISCAL
Rev. A | Page 14 of 28
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