ADE7761ARS-REF datasheet(14/28 Pages) AD

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ADE7761

Rev. A | Page 14 of 28

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.

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.

AGND

V1B

V1N

V1A

Figure 13. Typical Connection for Channel 1

*RB + VR = RF

VR*

RB*

RA*

V2P

V2N

V2P

V2N

±660mV

AGND

Figure 14. Typical Connection for Channel 2

VR1

MISCAL

V2N

REFIN/OUT

Figure 15. Typical Connection for MISCAL

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.

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.

2.5V

REFERENCE

INTERNAL

OSCILLATOR

ADE7761

DGND

RCLKIN

REFIN/OUT

Ω

ROSC

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.

....10100101....

1-BIT DAC

LATCHED

COMPAR-

ATOR

INTEGRATOR

VREF

MCLK

∫

ANALOG

LOW-PASS FILTER

DIGITAL

LOW-PASS FILTER

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

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