Datasheet ADE7761 Datasheet (Analog Devices)

Energy Metering IC with On-Chip

FEATURES

High accuracy active energy measurement IC, supports

IEC 687/61036 Less than 0.1% error over a dynamic range of 500 to 1 Supplies active power on the frequency outputs F1 and F2 High frequency output CF is intended for calibration and

supplies instantaneous active power Continuous monitoring of the phase and neutral current

allows fault detection in 2-wire distribution systems Current channels input level best suited for current

transformer sensors Uses the larger of the two currents (phase or neutral) to

bill—even during a fault condition Continuous monitoring of the voltage and current inputs

allows missing neutral detection Uses one current input (phase or neutral) to bill when

missing neutral is detected Two logic outputs (FAULT and REVP) can be used to indicate

a potential miswiring, fault, or missing neutral condition Direct drive for electromechanical counters and 2-phase

stepper motors (F1 and F2) Proprietary ADCs and DSP provide high accuracy over large

variations in environmental conditions and time Reference 2.5 V ± 8% (drift 30 ppm/°C typical) with external

overdrive capability Single 5 V supply, low power

FUNCTIONAL BLOCK DIAGRAM

AGND FAULT

MISCA

6 5

2.5V

REFERENCE

4kΩ

ADC

A>B

B>A

A<>B

MISSING NEUTRAL

INTERNAL

OSCILLATOR

HPF

GAIN ADJUST

Fault and Missing Neutral Detection

ADE7761

GENERAL DESCRIPTION

The ADE7761 is a high accuracy, fault tolerant, electrical energy measurement IC intended for use with 2-wire distribution systems. The part specifications surpass the accuracy requirements as quoted in the IEC61036 standard.

The only analog circuitry used on the ADE7761 is in the ADCs and reference circuit. All other signal processing (such as multiplication and filtering) is carried out in the digital domain. This approach provides superior stability and accuracy over extremes in environmental conditions and over time.

The ADE7761 incorporates a fault detection scheme similar to the ADE7751 by continuously monitoring both the phase and neutral currents. A fault is indicated when these currents differ by more than 6.25%.

(continued on Page 3)

15 18

ZERO CROSSING

DETECTION

MISSING NEUTRAL

DETECTION

DIGITAL-TO-FREQUENCY CONVERTER

POWER

SUPPLY MONITOR

ADE7761

SIGNAL PROCESSING

BLOCK

LPF

9 14 17 10 11 12

IN/OUT

16 18 19 20

F1F2CFREVPS0S1SCFDGNDRCLKINREF

04407-0-001

Figure 1.

Rev. A

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.

www.analog.com

ADE7761

TABLE OF CONTENTS

General Description......................................................................... 3

Active Power Calculation.......................................................... 15

Specifications..................................................................................... 4

Timing Characteristics..................................................................... 6

Absolute Maximum Ratings............................................................ 7

ESD Caution.................................................................................. 7

Terminology ......................................................................................8

Pin Configuration and Function Descriptions............................. 9

Typical Performance Characteristics ...........................................11

Operation......................................................................................... 13

Power Supply Monitor............................................................... 13

Analog Inputs..............................................................................13

Internal Oscillator ......................................................................14

Analog-to-Digital Conversion.................................................. 14

REVISION HISTORY

2/04—Changed from Rev. 0 to Rev. A.

Digital-to-Frequency Conversion............................................ 18

Transfer Function....................................................................... 18

Fault Detection ...........................................................................19

Missing Neutral Mode ............................................................... 20

Applications..................................................................................... 23

Interfacing to a Microcontroller for Energy Measurement.. 23

Selecting a Frequency for an Energy Meter Application....... 23

Negative Power Information..................................................... 24

Outline Dimensions....................................................................... 25

Ordering Guide .......................................................................... 25

Disclaimer........................................................................................ 26

Changes to Ordering Guide.......................................................... 25

1/04—Revision 0: Initial Version

Rev. A | Page 2 of 28

ADE7761

GENERAL DESCRIPTION

(continued from Page 1)

The ADE7761 incorporates a missing neutral detection scheme by continuously monitoring the input voltage. When a missing neutral condition is detected—no voltage input—the ADE7761 continues billing based on the active current signal (see the Missing Neutral Mode section). The missing neutral condition is indicated when the FAULT pin goes high.

The ADE7761 supplies average active power information on the low frequency outputs F1 and F2. The CF logic output gives instantaneous active power information.

The ADE7761 includes a power supply monitoring circuit on

supply pin. Internal phase matching circuitry ensures

the V

that the voltage and current channels are matched. An internal no-load threshold ensures that the ADE7761 does not exhibit any creep when there is no load.

Rev. A | Page 3 of 28

ADE7761

SPECIFICATIONS

VDD = 5 V ± 5%, AGND = DGND = 0 V, on-chip reference, on-chip oscillator, T

Table 1.

Parameter Value Unit Test Conditions/Comments

ACCURACY1

Measurement Error2 0.1 % of reading, typ Over a dynamic range of 500 to 1

Phase Error between Channels

(PF = 0.8 Capacitive) ±0.05 Degrees, max Phase lead 37°

(PF = 0.5 Inductive) ±0.05 Degrees, max Phase lag 60° AC Power Supply Rejection2 Output Frequency Variation 0.01 %, typ V1A = V1B = V2P = ±100 mV rms DC Power Supply Rejection2 Output Frequency Variation 0.01 %, typ V1A = V1B = V2P = ±100 mV rms

FAULT DETECTION

2, 3

See the Fault Detection section

Fault Detection Threshold

Inactive Input <> Active Input 6.25 %, typ (V1A or V1B active) Input Swap Threshold

Inactive Input <> Active Input 6.25 % of larger, typ (V1A or V1B active) Accuracy Fault Mode Operation

V1A Active, V1B = AGND 0.1 % of reading, typ Over a dynamic range of 500 to 1

V1B Active, V1A = AGND 0.1 % of reading, typ Over a dynamic range of 500 to 1 Fault Detection Delay 3 Seconds, typ Swap Delay 3 Seconds, typ

MISSING NEUTRAL MODE

2, 4

See the Missing Neutral Detection section

Missing Neutral Detection Threshold

V2P − V2N 59.4 mV peak, min Accuracy Missing Neutral Mode

V1A Active, V1B = V2P = AGND 0.1 % of reading, typ Over a dynamic range of 500 to 1

V1B Active, V1A = V2P = AGND 0.1 % of reading, typ Over a dynamic range of 500 to 1 Missing Neutral Detection Delay 3 Seconds, typ

ANALOG INPUTS V1A − V1N, V1B − V1N, V2P − V2N

Maximum Signal Levels ±660 mV peak, max Differential input 660 mV peak, max Differential input MISCAL − V2N Input Impedance (DC) 400 kΩ, min Bandwidth (−3 dB) 7 kHz, typ ADC Offset Error2 10 mV, max Uncalibrated error, see the Terminology section for details Gain Error ±4 %, typ External 2.5 V reference

REFERENCE INPUT

REF

Input Voltage Range 2.7 V, max 2.5 V + 8%

IN/OUT

2.3 V, min 2.5 V − 8% Input Impedance 4 kΩ, min Input Capacitance 10 pF, max

ON-CHIP REFERENCE

Reference Error ±200 mV, max Temperature Coefficient 30 ppm/°C, typ Current Source 20 µA, min

ON-CHIP OSCILLATOR

Oscillator Frequency 450 kHz Oscillator Frequency Tolerance ±12 % of reading, typ Temperature Coefficient 30 ppm/°C, typ

See footnotes on next page.

MIN

to T

= –40°C to +85°C.

MAX

Rev. A | Page 4 of 28

ADE7761

Parameter Value Unit Test Conditions/Comments

LOGIC INPUTS5

SCF, S1, and S0

Input High Voltage, V Input Low Voltage, V Input Current, IIN ±3 µA, max Typical 10 nA, VIN = 0 V to VDD Input Capacitance, CIN 10 pF, max

LOGIC OUTPUTS5

CF, REVP, and FAULT

Output High Voltage, VOH 4 V, min VDD = 5 V ± 5% Output Low Voltage, VOH 1 V, max VDD = 5 V ± 5%

F1 and F2

Output High Voltage, VOH 4 V, min VDD = 5 V ± 5%, I Output Low Voltage, VOH 1 V, max VDD = 5 V ± 5%, I

POWER SUPPLY For specified performance

VDD 4.75 V, min 5 V − 5%

5.25 V, max 5 V + 5% VDD 4 mA, max

See plots in the Typical Performance Characteristics section.

See the Terminology section for explanation of specifications.

See the Fault Detection section for explanation of fault detection functionality.

See the Missing Neutral Detection section for explanation of missing neutral detection functionality.

Sample tested during initial release and after any redesign or process change that may affect this parameter.

2.4 V, min VDD = 5 V ± 5%

INH

0.8 V, max VDD = 5 V ± 5%

INL

SOURCE

= 10 mA

SINK

= 10 mA

Rev. A | Page 5 of 28

ADE7761

TIMING CHARACTERISTICS

VDD = 5 V ± 5%, AGND = DGND = 0 V, on-chip reference, on-chip oscillator, T Sample tested during initial release and after any redesign or process change that may affect this parameter. See Figure 2.

Table 2.

Parameter Value Unit Test Conditions/Comments

120 ms F1 and F2 Pulse Width (Logic High).

t2 See Table 6 s Output Pulse Period. See the Transfer Function section. t3 1/2 t2 s Time between F1 Falling Edge and F2 Falling Edge.

90 ms CF Pulse Width (Logic High).

t5 See Table 7 s CF Pulse Period. See the Transfer Function section. t6 CLKIN/4 s Minimum Time between F1 and F2 Pulse.

The pulse widths of F1, F2, and CF are not fixed for higher output frequencies. See the Transfer Function section.

to T

MIN

Figure 2. Timing Diagram for Frequency Outputs

= –40°C to +85°C.

MAX

04407-0-002

Rev. A | Page 6 of 28

ADE7761

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 3.

Parameter Rating

VDD to AGND −0.3 V to +7 V

Analog Input Voltage to AGND

, V

V Reference Input Voltage to AGND −0.3 V to VDD + 0.3 V Digital Input Voltage to DGND −0.3 V to VDD + 0.3 V Digital Output Voltage to DGND −0.3 V to VDD + 0.3 V Operating Temperature Range

Industrial −40°C to +85°C Storage Temperature Range −65°C to +150°C Junction Temperature 150°C 20-Lead SSOP, Power Dissipation 450 mW θJA Thermal Impedance 112°C/W Lead Temperature, Soldering

Vapor Phase (60 s) 215°C

Infrared (15 s) 220°C

, V1N, V2N, V2P, MISCAL

1AP

1BP

−6 V to +6 V

Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

ESD CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.

Rev. A | Page 7 of 28

ADE7761

TERMINOLOGY

Measurement Error

The error associated with the energy measurement made by the ADE7761 is defined by the following formula:

Erro

Percentag

⎛ ⎜

⎜ ⎝

Phase Error between Channels

The high-pass filter (HPF) in the current channel has a phase lead response. To offset this phase response and equalize the phase response between channels, a phase correction network is also placed in the current channel. The phase correction network ensures a phase match between the current channels and voltage channels to within ±0.1° over a range of 45 Hz to 65 Hz and ±0.2° over a range 40 Hz to 1 kHz.

Power Supply Rejection

This quantifies the ADE7761 measurement error as a percentage of reading when the power supplies are varied. For the ac PSR measurement, a reading at nominal supplies (5 V) is taken. A second reading is obtained with the same input signal levels when an ac (175 mV rms/100 Hz) signal is introduced onto the supplies. Any error introduced by this ac signal is expressed as a percentage of reading (see the Measurement Error definition above).

⎞

7761

EnergyTrue

EnergyTrueADEbyregisteredEnergy

− ×

⎟

%100

⎟ ⎠

For the dc PSR measurement, a reading at nominal supplies (5 V) is taken. A second reading is obtained with the same input signal levels when the power supplies are varied ±5%. Any error introduced is again expressed as a percentage of reading.

ADC Offset Error

This refers to the dc offset associated with the analog inputs to the ADCs. It means that with the analog inputs connected to AGND, the ADCs still see a dc analog input signal. The magnitude of the offset depends on the input range selection (see the Typical Performance Characteristics section). However, when HPFs are switched on, the offset is removed from the current channels and the power calculation is not affected by this offset.

Gain Error

The gain error in the ADE7761 ADCs is defined as the difference between the measured output frequency (minus the offset) and the ideal output frequency. The difference is expressed as a percentage of the ideal frequency, which is obtained from the transfer function (see the Transfer Function section).

Rev. A | Page 8 of 28

ADE7761

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

Table 4. Pin Function Descriptions

Pin No. Mnemonic Description

1 VDD

Power Supply. This pin provides the supply voltage for the digital circuitry in the ADE7761. The supply voltage should be maintained at 5 V ± 5% for specified operation. This pin should be decoupled with a 10 µF capacitor in parallel with a ceramic 100 nF capacitor.

2, 3 V1A, V1B

Analog Inputs for Channel 1 (Current Channel). These inputs are fully differential voltage inputs with maximum differential input signal levels of ±660 mV with respect to V maximum signal level at these pins is ±1 V with respect to AGND. Both inputs have internal ESD protection circuitry, and an overvoltage of ±6 V can also be sustained on these inputs without risk of permanent damage.

4 V1N

Negative Input Pin for Differential Voltage Inputs V ±1 V with respect to AGND. The input has internal ESD protection circuitry, and an overvoltage of ±6 V can also be sustained on these inputs without risk of permanent damage. The input should be directly connected to the burden resistor and held at a fixed potential,that is, AGND. See the Analog Inputs section.

5 V2N

Negative Input Pin for Differential Voltage Inputs V ±1 V with respect to AGND. The input has internal ESD protection circuitry, and an overvoltage of ±6 V can also be sustained on these inputs without risk of permanent damage. The input should be held at a fixed potential, that is, AGND. See the Analog Inputs section.

6 V2P

Analog Inputs for Channel 2 (Voltage Channel). This input is fully differential voltage input with maximum differential input signal levels of ±660 mV with respect to V maximum signal level at these pins is ±1 V with respect to AGND. This input has internal ESD protection circuitry, and an overvoltage of ±6 V can also be sustained on these inputs without risk of permanent damage.

7 MISCAL

Analog Input for Missing Neutral Calibration. This pin can be used to calibrate the CF-F1-F2 frequencies in the missing neutral condition. This input is fully differential voltage input with maximum differential input signal levels of +660 mV with respect to V this pin is ±1 V with respect to AGND. This input has internal ESD protection circuitry, and an overvoltage of ±6 V can also be sustained on these inputs without risk of permanent damage.

8 AGND

This pin provides the ground reference for the analog circuitry in the ADE7761, that is, ADCs and reference. This pin should be tied to the analog ground plane of the PCB. The analog ground plane is the ground reference for all analog circuitry such as antialiasing filters, and current and voltage transducers. For good noise suppression, the analog ground plane should be connected only to the digital ground plane at the DGND pin.

9 REF

IN/OUT

This pin provides access to the on-chip voltage reference. The on-chip reference has a nominal value of

2.5 V ± 8% and a typical temperature coefficient of 30 ppm/°C. An external reference source can also be connected at this pin. In either case, this pin should be decoupled to AGND with a 1 μF ceramic capacitor and 100 nF ceramic capacitor.

10 SCF

Select Calibration Frequency. This logic input is used to select the frequency on the calibration output CF. Table 6 shows how the calibration frequencies are selected.

11, 12 S1, S0

These logic inputs are used to select one of four possible frequencies for the digital-to-frequency conversion. This offers the designer greater flexibility when designing the energy meter. See the Selecting a Frequency for an Energy Meter Application section.

7 8 9

ADE7761

TOP VIEW

(Not to Scale)

MISCAL

REF

V V

AGND

IN/OUT

SCF

Figure 3. Pin Configuration (SSOP)

20 19 18 17 16 15 14 13 12 11

F1 F2 CF DGND REVP FAULT RCLKIN INT S0 S1

04407-0-003

for specified operation. The

and V1B. The maximum signal level at this pin is

and MISCAL. The maximum signal level at this pin is

for specified operation. The

for specified operation. The maximum signal level at

Rev. A | Page 9 of 28

ADE7761

Pin No. Mnemonic Description

13 INT This pin is internally used and should be connected to DGND. 14 RCLKIN

15 FAULT

16 REVP

17 DGND

18 CF

19, 20 F2, F1

To enable the internal oscillator as a clock source on the chip, a precise low temperature drift resistor at nominal value of 6.2 kΩ must be connected from this pin to DGND.

This logic output goes active high when a fault or missing neutral condition occurs. A fault is defined as a condition under which the signals on V condition is defined when the chip is powered up with no voltage at the input. The logic output is reset to zero when a fault or missing neutral condition is no longer detected. See the Fault Detection section and the Missing Neutral Mode section.

This logic output goes logic high when negative power is detected, that is, when the phase angle between the voltage and current signals is greater than 90°. This output is not latched and is reset when positive power is once again detected. The output goes high or low at the same time as a pulse is issued on CF.

This pin provides the ground reference for the digital circuitry in the ADE7761, that is, multiplier, filters, and digital-to-frequency converter. This pin should be tied to the digital ground plane of the PCB. The digital ground plane is the ground reference for all digital circuitry such as counters (mechanical and digital), MCUs, and indicator LEDs. For good noise suppression, the analog ground plane should be connected only to the digital ground plane at the DGND pin.

Calibration Frequency Logic Output. The CF logic output, active high, gives instantaneous active power information. This output is intended to be used for operational and calibration purposes. See the Digitalto-Frequency Conversion section.

Low Frequency Logic Outputs. F1 and F2 supply average active power information. The logic outputs can be used to directly drive electromechanical counters and 2-phase stepper motors.

and V1B differ by more than 6.25%. A missing neutral

Rev. A | Page 10 of 28

ADE7761

TYPICAL PERFORMANCE CHARACTERISTICS

1.0 PF = 1

ON-CHIP REFERENCE

–0.2

% ERROR

–0.4

–0.6

–0.8

–1.0

0.8

0.6

0.4

0.2

CURRENT (% of Full Scale)

–40°C

+25°C

+85°C

Figure 4. Active Power Error as a Percentage of Reading with

Internal Reference

1.5 PF = 0.5

ON-CHIP REFERENCE

% ERROR

–0.5

1.0

0.5

–40°C; PF = 0.5

+25°C; PF = 1

+85°C; PF = 0.5

+25°C; PF = 0.5

100.00.1 1.0 10.0

04407-0-037

1.0 PF = 1

ON-CHIP REFERENCE

–0.2

% ERROR

–0.4

–0.6

–0.8

–1.0

0.8

0.6

0.4

0.2

5.25V

5.00V

4.75V

CURRENT (% of Full Scale)

Figure 6. Active Power Error as a Percentage of Reading

over Power Supply with Internal Reference

2.0 ON-CHIP REFERENCE

1.5

% ERROR

–0.5

–1.0

–1.5

1.0

0.5

+25°C

+85°C

–40°C

100.00.1 1.0 10.0

04434-0-039

–1.0

CURRENT (% of Full Scale)

Figure 5. Active Power Error as a Percentage of Reading over

Power Factor with Internal Reference

100.00.1 1.0 10.0

04407-0-038

–2.0

CURRENT (% of Full Scale)

100.00.1 1.0 10.0

04434-0-040

Figure 7. Ampere Hour Error as a Percentage of Reading in

Missing Neutral Mode with Internal Reference

Rev. A | Page 11 of 28

ADE7761

10µF

560kΩ

100kΩ

1kΩ

33nF

1kΩ

33nF

1kΩ

33nF

1kΩ

33nF

220V

40A TO 80mA

RB = 18Ω

1MΩ

33nF1kΩ

100nF

MISCAL

ADE7761

AGND DGNDINT

RCLKIN

REF

FAULT

S0 S1

SCF

IN/OUT

1713 8

2kΩ

6.2kΩ

PS2501-1

TO FREQ. COUNTER

10kΩ

100nF

10µF

04407-0-036

Figure 8. Test Circuit for Performances Curves

Rev. A | Page 12 of 28

ADE7761

–

OPERATION

POWER SUPPLY MONITOR

The ADE7761 contains an on-chip power supply monitor. The power supply (V

) is continuously monitored by the ADE7761.

If the supply is less than 4 V ± 5%, the ADE7761 goes into an inactive state, that is, no energy is accumulated and the CF, F1, and F2 outputs are disabled. This is useful to ensure correct device operation at power-up and during power-down. The power supply monitor has built-in hysteresis and filtering. This gives a high degree of immunity to false triggering due to noisy supplies.

The power supply and decoupling for the part should be such that the ripple at V

does not exceed 5 V ± 5% as specified for

normal operation.

5V 4V

ADE7761

REVP - FAULT - CF

F1 - F2 OUTPUTS

INACTIVE ACTIVE

Figure 9. On-Chip Power Supply Monitoring

TIME

INACTIVE

ANALOG INPUTS

Channel V1 (Current Channel)

The voltage outputs from the current transducers are connected to the ADE7761 here. Channel V1 has two voltage inputs, V and V

. These inputs are fully differential with respect to V1N.

However, at any one time, only one is selected to perform the power calculation (see the Fault Detection section).

The maximum peak differential signal on V

1A–V1N

and V1B–V1N

is ±660 mV.

Figure 10 shows the maximum signal levels on V

, V1B, and V1N.

The differential voltage signal on the inputs must be referenced to a common mode (usually AGND).

+660mV + V

–660mV + V

DIFFERENTIAL INPUT A

, V

±660mV MAX PEAK

COMMON MODE

±100mV MAX

AGND

DIFFERENTIAL INPUT B

±660mV MAX PEAK

Figure 10. Maximum Signal Levels, Channel 1

04407-0-010

04407-0-011

Channel V2 (Voltage Channel)

The output of the line voltage transducer is connected to the ADE7761 at this analog input. Channel V2 is a single-ended voltage input. The maximum peak differential signal on Channel 2 is ±660 mV with respect to V

. Figure 11 shows the

maximum signal levels that can be connected to Channel 2.

+660mV + V

660mV + V

DIFFERENTIAL INPUT

±660mV MAX PEAK

COMMON MODE

±100mV MAX

Figure 11. Maximum Signal Levels, Channel 2

The differential voltage V2P–V2N must be referenced to a common mode (usually AGND). The analog inputs of the ADE7761 can be driven with common-mode voltages of up to 100 mV with respect to AGND. However, the best results are achieved using a common mode equal to AGND.

MISCAL Input

The input for the power calibration in missing neutral mode is connected to the ADE7761 at this analog input. MISCAL is a single-ended voltage input. It is recommended to use a dc signal derived from the voltage reference to drive this pin. The maximum peak differential signal on MISCAL is +660 mV with respect to V

. Figure 12 shows the maximum signal levels that

can be connected to the MISCAL pin.

+660mV + V

MISCAL

DIFFERENTIAL INPUT

±660mV MAX PEAK

COMMON MODE

±100mV MAX

Figure 12. Maximum Signal Levels, MISCAL

MISCAL

AGND

The differential voltage MISCAL–V2N must be referenced to a common mode (usually AGND). The analog inputs of the ADE7761 can be driven with common-mode voltages of up to 100 mV with respect to AGND. However, best results are achieved using a common mode equal to AGND.

Typical Connection Diagrams

Figure 13 shows a typical connection diagram for Channel V1. The analog inputs are being used to monitor both the phase and neutral currents. Because of the large potential difference between the phase and neutral, two current transformers (CTs) must be used to provide the isolation. Note that both CTs are referenced to analog ground (AGND); the common-mode

04407-0-012

04407-0-013

Rev. A | Page 13 of 28

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.

Figure 15 shows a typical connection for MISCAL input. The voltage reference input (REF 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 + VR reference is used, the values of RC, RD, and VR to limit the current sourced by the internal reference sourcing current to below the specified 20 µA. Therefore, because V internal = 2.5 V, RC + RD + VR

INIP

AGND

PHASE

REF

NEUTRAL

Figure 13. Typical Connection for Channel 1

±660mV

NEUTRAL

PHASE

RA*

NEUTRAL

PHASE

*RB + VR = RF

AGND

RB*

VR*

Figure 14. Typical Connection for Channel 2

IN/OUT

VR1

Figure 15. Typical Connection for MISCAL

) is used as a dc reference to

IN/OUT

. When the internal

must be chosen

> 600 kΩ.

MISCAL

REF

04407-0-015

04407-0-016

04407-0-014

INTERNAL OSCILLATOR

The nominal internal oscillator frequency is 450 kHz when used with the recommended R

resistor value of 6.2 kΩ

OSC

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 R

resistor value between 5 kΩ and 12 kΩ, it

OSC

is recommended to choose a value within the range of the nominal value.

The output frequencies on CF, F1, and F2 are directly proportional to the internal oscillator frequency; therefore, the resistor R

must have a low tolerance and low temperature drift. A low

OSC

tolerance resistor limits the variation of the internal oscillator frequency. Small variation of the clock frequency and consequently 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

Ω

IN/OUT

INTERNAL

OSCILLATOR

14 17

OSC

DGNDRCLKINREF

Figure 16. Internal Oscillator Connection

04407-0-017

2.5V

REFERENCE

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.

MCLK

ANALOG

LOW-PASS FILTER

INTEGRATOR

∫

REF

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

LATCHED COMPARATOR

....10100101....

DIGITAL

LOW-PASS FILTER

1 24

04407-0-019

Rev. A | Page 14 of 28

ADE7761

)

output (and, therefore, the bit stream) approaches that of the input signal level. For any given input value in a single sampling interval, the data from the 1-bit ADC is virtually meaningless. Only when a large number of samples are averaged is a meaningful result obtained. This averaging is carried out in the second part of the ADC, the digital low-pass filter. By averaging a large number of bits from the modulator, the low-pass filter can produce 24-bit data words that are proportional to the input signal level.

The Σ-Δ converter uses two techniques to achieve high resolution from what is essentially a 1-bit conversion technique. The first is oversampling, which means that the signal is sampled at a rate (frequency) that is many times higher than the bandwidth of interest. For example, the sampling rate in the ADE7761 is CLKIN (450 kHz) and the band of interest is 40 Hz to 1 kHz. Oversampling has the effect of spreading the quantization noise (noise due to sampling) over a wider bandwidth. With the noise spread more thinly over a wider bandwidth, the quantization noise in the band of interest is lowered (see Figure 18).

However, oversampling alone is not an efficient enough method to improve the signal-to-noise ratio (SNR) in the band of interest. For example, an oversampling ratio of 4 is required just to increase the SNR by only 6 dB (1 bit). To keep the oversampling ratio at a reasonable level, it is possible to shape the quantization noise so that the majority of the noise lies at the higher frequencies. This is what happens in the Σ-Δ modulator; the noise is shaped by the integrator, which has a high-pass type response for the quantization noise. The result is that most of the noise is at the higher frequencies where it can be removed by the digital low-pass filter. This noise shaping is also shown in Figure 18.

ANTIALIAS FILTER (RC

IGNAL

NOISE

0 1kHz 225kHz 450kHz

IGNAL

NOISE

0 1kHz 225kHz 450kHz

DIGITAL FILTER

FREQUENCY (Hz)

HIGH RESOLUTION

OUTPUT FROM

DIGITAL LFP

FREQUENCY (Hz)

Figure 18. Noise Reduction Due to Oversampling and

Noise Shaping in the Analog Modulator

SAMPLING FREQUENCY

SHAPED NOISE

04407-0-020

Antialias Filter

Figure 18 also shows an analog low-pass filter (RC) on input to the modulator. This filter is present to prevent aliasing. Aliasing is an artifact of all sampled systems, which means that frequency components in the input signal to the ADC that are higher than half the sampling rate of the ADC appear in the sampled signal frequency below half the sampling rate. Figure 19 illustrates the effect.

In Figure 19, frequency components (arrows shown in black) above half the sampling frequency (also known as the Nyquist frequency), that is, 225 kHz, are imaged or folded back down below 225 kHz (arrows shown in gray). This happens with all ADCs no matter what the architecture. In the example shown, only frequencies near the sampling frequency (450 kHz) move into the band of interest for metering (40 Hz to 1 kHz). This fact allows the use of a very simple low-pass filter to attenuate these frequencies (near 250 kHz) and thereby prevent distortion in the band of interest. A simple RC filter (single pole) with a corner frequency of 10 kHz produces an attenuation of approximately 33 dB at 450 kHz (see Figure 19). This is sufficient to eliminate the effects of aliasing.

ANTIALIASING EFFECTS

SAMPLING

IMAGE

FREQUENCIES

0 1kHz 225kHz 450kHz

FREQUENCY (Hz)

Figure 19. ADC and Signal Processing in Current Channel or Voltage Channel

FREQUENCY

04407-0-021

ACTIVE POWER CALCULATION

The ADCs digitize the voltage signals from the current and voltage transducers. A high-pass filter in the current channel removes any dc component from the current signal. This eliminates any inaccuracies in the active power calculation due to offsets in the voltage or current signals (see the HPF and Offset Effects section).

The active power calculation is derived from the instantaneous power signal. The instantaneous power signal is generated by a direct multiplication of the current and voltage signals. To extract the active power component (dc component), the instantaneous power signal is low-pass filtered. Figure 20 illustrates the instantaneous active power signal and shows how the active power information can be extracted by low-pass filtering the instantaneous power signal. This scheme correctly calculates active power for nonsinusoidal current and voltage waveforms at all power factors. All signal processing is carried out in the digital domain for superior stability over temperature and time.

Rev. A | Page 15 of 28

ADE7761

∞

The low frequency output of the ADE7761 is generated by accumulating this active power information. This low frequency inherently means a long accumulation time between output pulses. The output frequency is, therefore, proportional to the average active power. This average active power information can in turn be accumulated (for example, by a counter) to generate active energy information. Because of its high output frequency and therefore shorter integration time, the CF output is proportional to the instantaneous active power. This is useful for system calibration purposes that would take place under steady load conditions.

DIGITAL-TO-

CH1

CH2

V× I

TIME

ADC

HPF

MULTIPLIER

ADC

INSTANTANEOUS

POWER SIGNAL –p(t)

p(t) = i(t).v(t) WHERE: v(t) = V × cos(ϖt) i(t) = I × cos(ϖt)

V× I

p(t) =

LPF

INSTANTANEOUS

ACTIVE POWER SIGNAL

V× I

{1 + cos (2ϖt)}

FREQUENCY

DIGITAL-TO-

FREQUENCY

F1 F2

Figure 20. Signal Processing Block Diagram

Power Factor Considerations

The method used to extract the active power information from the instantaneous power signal (by low-pass filtering) is still valid even when the voltage and current signals are not in phase. Figure 21 displays the unity power factor condition and a displacement power factor (DPF = 0.5), that is, current signal lagging the voltage by 60°. If one assumes that the voltage and current waveforms are sinusoidal, the active power component of the instantaneous power signal (dc term) is given by

04407-0-022

V× I

× cos(60°)

INSTANTANEOUS POWER SIGNAL

CURRENT VOLTAGE

VOLTAGE

INSTANTANEOUS POWER SIGNAL

60°

INSTANTANEOUS ACTIVE POWER SIGNAL

CURRENT

Figure 21. Active Power Calculation over PF

Nonsinusoidal Voltage and Current

The active power calculation method also holds true for nonsinusoidal current and voltage waveforms. All voltage and current waveforms in practical applications have some harmonic content. Using the Fourier transform, instantaneous voltage and current waveforms can be expressed in terms of their harmonic content:

)sin(2)(

thVVtv α+ω××+=

∑

0hh

≠

(1)

where:

v(t) is the instantaneous voltage.

is the average value.

is the rms value of voltage harmonic h.

is the phase angle of the voltage harmonic.

04407-0-023

(2)

)sin(2)(

(V × I/2) × cos(60°)

∑

0hh

≠

thIIti β+ω××+=

This is the correct active power calculation.

where:

i(t) is the instantaneous current.

is the dc component.

is the rms value of current harmonic h.

is the phase angle of the current harmonic.

Rev. A | Page 16 of 28

ADE7761

Using Equations 1 and 2, the active power P can be expressed in terms of its fundamental active power (P power (P

PPP +=

) and harmonic active

where:

IVP

Φ×= )cos(

1111

β−α=Φ

111

(3)

and

∞

IVP

∑

β−α=Φ

hhh

Φ××=

)cos(

hhh

(4)

As can be seen from Equation 4, a harmonic active power component is generated for every harmonic, provided that harmonic is present in both the voltage and current waveforms. The power factor calculation has previously been shown to be accurate in the case of a pure sinusoid; the harmonic active power must, therefore, also correctly account for power factor, because it is made up of a series of pure sinusoids.

Note that the input bandwidth of the analog inputs is 7 kHz with the internal oscillator frequency of 450 kHz.

HPF and Offset Effects

Equation 5 shows the effect of offsets on the active power calculation. Figure 22 shows the effect of offsets on the active power calculation in the frequency domain.

tItV

)()(

=×

tIItVV

))cos(())cos((

1010

+×

=ω×+×ω×+

0110

(5)

tIVtIV

)cos()cos(

ω××+ω××+

V1× I

Figure 22. Effect of Channel Offsets on the Active Power Calculation

0.30

0.25

0.20

0.15

0.10

0.05

PHASE (Degrees)

–0.05

–0.10

0 100

Figure 23. Phase Error between Channels (0 Hz to 1 kHz)

0.30

0.25

0.20

0.15

0.10

DC COMPONENT (INCLUDING ERROR TERM) IS EXTRACTED BY THE LPF FOR ACTIVE POWER CALCULATION

V1× I

V0× I

200 300 400 500 600 700 800 900 1000

FREQUENCY (RAD/S)

FREQUENCY (Hz)

04407-0-024

04407-0-025

As can be seen from Equation 5 and Figure 22, an offset on Channel 1 and Channel 2 contributes a dc component after multiplication. Because this dc component is extracted by the LPF and used to generate the active power information, the offsets contribute a constant error to the active power calculation. This problem is easily avoided in the ADE7761 with the HPF in Channel 1. By removing the offset from at least one channel, no error component can be generated at dc by the multiplication. Error terms at cos(ωt) are removed by the LPF and the digital-to-frequency conversion (see the Digital-toFrequency Conversion section).

The HPF in Channel 1 has an associated phase response that is compensated for on-chip. Figure 23 and Figure 24 show the phase error between channels with the compensation network activated. The ADE7761 is phase compensated up to 1 kHz as shown, which ensures correct active harmonic power calculation even at low power factors.

Rev. A | Page 17 of 28

0.05

PHASE (Degrees)

–0.05

–0.10

45 50 55 60 65 70

FREQUENCY (Hz)

04407-0-026

Figure 24. Phase Error between Channels (40 Hz to 70 Hz)

ADE7761

)

DIGITAL-TO-FREQUENCY CONVERSION

As previously described, the digital output of the low-pass filter after multiplication contains the active power information. However, because this LPF is not an ideal “brick wall” filter implementation, the output signal also contains attenuated components at the line frequency and its harmonics, that is, cos(hωt), where h = 1, 2, 3, …, and so on. The magnitude response of the filter is given by

)(ffH

For a line frequency of 50 Hz, this gives an attenuation of the 2ω (100 Hz) component of approximately –26.9 dB. The dominating harmonic is at twice the line frequency, cos(2ωt), due to the instantaneous power signal.

Figure 25 shows the instantaneous active power signal output of the LPF, which still contains a significant amount of instantaneous power information, cos(2ωt). This signal is then passed to the digital-to-frequency converter, where it is integrated (accumulated) over time to produce an output frequency. This accumulation of the signal suppresses or averages out any nondc components in the instantaneous active power signal. The average value of a sinusoidal signal is zero. Therefore, the frequency generated by the ADE7761 is proportional to the average active power.

Figure 25 also shows the digital-to-frequency conversion for steady load conditions: constant voltage and current. As can be seen in Figure 25, the frequency output CF varies over time, even under steady load conditions. This frequency variation is primarily due to the cos(2ωt) component in the instantaneous active power signal.

MULTIPLIER

LPF TO EXTRACT ACTIVE POWER (DC TERM)

0 ϖ 2ϖ

FREQUENCY (Rad/s)

LPF

(6)

)Hz5.4/(1

DIGITAL-TO-

FREQUENCY

DIGITAL-TO-

FREQUENCY

F1 F2

FOUT

TIME

FREQUENCY FREQUENCY

TIME

The output frequency on CF can be up to 2048 times higher than the frequency on F1 and F2. This higher output frequency is generated by accumulating the instantaneous active power signal over a much shorter time while converting it to a frequency. This shorter accumulation period means less averaging of the cos(2ωt) component. As a consequence, some of this instantaneous power signal passes through the digital-tofrequency conversion. This is not a problem in the application.

Where CF is used for calibration purposes, the frequency should be averaged by the frequency counter, which removes any ripple. If CF is being used to measure energy, such as in a microprocessor-based application, the CF output should also be averaged to calculate power. Because the outputs F1 and F2 operate at a much lower frequency, a lot more averaging of the instantaneous active power signal is carried out. The result is a greatly attenuated sinusoidal content and a virtually ripple-free frequency output.

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7761 calculates the product of two voltage signals (on Channel 1 and Channel 2) and then low-pass filters this product to extract active power information. This active power information is then converted to a frequency. The frequency information is output on F1 and F2 in the form of active high pulses. The pulse rate at these outputs is relatively low, for example, 0.34 Hz maximum for ac signals with S0 = S1 = 0 (see Table 7). This means that the frequency at these outputs is generated from active power information accumulated over a relatively long period of time. The result is an output frequency that is proportional to the average active power. The averaging of the active power signal is implicit to the digital-to-frequency conversion. The output frequency or pulse rate is related to the input voltage signals by the following equation:

2170.5

××

rmsrms

−

FrequencyFF

=−

REF

where:

− F2 Frequency is the output frequency on F1 and F2 (Hz).

is the differential rms voltage signal on Channel 1 (V).

rms

is the differential rms voltage signal on Channel 2 (V).

rms

is the reference voltage (2.5 V ± 8%) (V).

REF

is one of four possible frequencies selected by using the

1–4

logic inputs S0 and S1 (see Table 5).

(7)

INSTANTANEOUS ACTIVE POWER SIGNAL (FREQUENCY DOMAIN

Figure 25. Active Power to Frequency Conversion

04407-0-027

Rev. A | Page 18 of 28

ADE7761

Table 5. F

S1 S0 F

0 0 1.72 OSC/218 0 1 3.44 OSC/217 1 0 6.86 OSC/216 1 1 13.7 OSC/215

Values are generated using the nominal frequency of 450 kHz.

are a binary fraction of the master clock and, therefore, vary with the

1–4

internal oscillator frequency (OSC).

Frequency Output CF

The pulse output calibration frequency (CF) is intended for use during calibration. The output pulse rate on CF can be up to 2048 times the pulse rate on F1 and F2. The lower the F frequency selected, the higher the CF scaling. Table 6 shows how the two frequencies are related, depending on the states of the logic inputs S0, S1, and SCF. Because of its relatively high pulse rate, the frequency at this logic output is proportional to the instantaneous active power. As with F1 and F2, the frequency is derived from the output of the low-pass filter after multiplication. However, because the output frequency is high, this active power information is accumulated over a much shorter time. Therefore, less averaging is carried out in the digital-to-frequency conversion. With much less averaging of the active power signal, the CF output is much more responsive to power fluctuations (see Figure 20).

Table 6. Relationship between CF and F1, F2 Frequency Outputs

SCF S1 S0 F

1 0 0 1.72 128 × F1, F2 0 0 0 1.72 64 × F1, F2 1 0 1 3.44 64 × F1, F2 0 0 1 3.44 32 × F1, F2 1 1 0 6.86 32 × F1, F2 0 1 0 6.86 16 × F1, F2 1 1 1 13.7 16 × F1, F2 0 1 1 13.7 2048 × F1, F2

Example

In this example, if ac voltages of ±660 mV peak are applied to V1 and V2, then the expected output frequency on CF, F1, and F2 is calculated as follows:

Frequency Selection

1–4

(Hz)1 OSC/CLKIN2

1–4

(Hz) CF Frequency Output

1–4

= 1.7 Hz, SCF = S1 = S0 = 0

1–4

= rms of 660 mV peak ac = 0.66/√2 V

rms

= rms of 660 mV peak ac = 0.66/√2 V

rms

1–4

Note that if the on-chip reference is used, actual output frequencies may vary from device to device due to a reference tolerance of ±8%.

FrequencyFF

=−

FFFrequencyCF

××

Hz0.2264

=×−=

Hz72.166.066.070.5

5.222

Hz34.0

As can be seen from these two example calculations, the maximum output frequency for ac inputs is always half of that for dc input signals. Table 7 shows a complete listing of all maximum output frequencies for ac signals.

Table 7. Maximum Output Frequency on CF, F1, and F2 for AC Inputs

CF to

SCF S1 S0

1 0 0 0.34 43.52 128 0 0 0 0.34 21.76 64 1 0 1 0.68 43.52 64 0 0 1 0.68 21.76 32 1 1 0 1.36 43.52 32 0 1 0 1.36 21.76 16 1 1 1 2.72 43.52 16 0 1 1 2.72 5570 2048

F1, F2 Maximum Frequency (Hz)

CF Maximum Frequency (Hz)

F1 Ratio

FAULT DETECTION

The ADE7761 incorporates a novel fault detection scheme that warns of fault conditions and allows the ADE7761 to continue accurate billing during a fault event. The ADE7761 does this by continuously monitoring both the phase and neutral (return) currents. A fault is indicated when these currents differ by more than 6.25%. However, even during a fault, the output pulse rate on F1 and F2 is generated using the larger of the two currents. Because the ADE7761 looks for a difference between the voltage signals on V ducers be closely matched.

On power-up, the output pulse rate of the ADE7761 is proportional to the product of the voltage signals on V Channel 2. If there is a difference of greater than 6.25% between V

and V1B on power-up, the fault indicator (FAULT) becomes

active after about 1 s. In addition, if V ADE7761 selects V automatically disabled when the voltage signal on Channel 1 is less than 0.3% of the full-scale input range. This eliminates false detection of a fault due to noise at light loads.

and V1B, it is important that both current trans-

and

is greater than V1A, the

as the input. The fault detection is

= 2.5 V (nominal reference value)

REF

Rev. A | Page 19 of 28

ADE7761

Fault with Active Input Greater than Inactive Input

If V1A is the active current input (that is, being used for billing), and the voltage signal on V of V

, the fault indicator becomes active. Both analog inputs are

filtered and averaged to prevent false triggering of this logic output. As a consequence of the filtering, there is a time delay of approximately 3 s on the logic output FAULT after the fault event. The FAULT logic output is independent of any activity on outputs F1 or F2. Figure 26 shows one condition under which FAULT becomes active. Because V still greater than V swap to the V

V1B < 93.75% OF V

FAULT

6.25% OF ACTIVE INPUT

Figure 26. Fault Conditions for Active Input Greater than Inactive Input

, billing is maintained on V1A, that is, no

input occurs. V1A remains the active input.

AGND

Fault with Inactive Input Greater than Active Input

Figure 27 illustrates another fault condition. If the difference between V

, the inactive input, and V1A, the active input (that is,

being used for billing), becomes greater than 6.25% of V FAULT indicator goes active, and there is also a swap over to the V

input. The analog input V1B becomes the active input. Again,

there is a time constant of about 3 s associated with this swap. V

does not swap back to being the active channel until V1A is

greater than V

and the difference between V1A and V1B—in this

order—becomes greater than 6.25% of V FAULT indicator becomes inactive as soon as V

6.25% of V between V

. This threshold eliminates potential chatter

and V1B.

AGND

V1A < 93.75% OF V

FAULT + SWAP

(inactive input) falls below 93.75%

is the active input and it is

ACTIVE POINT – INACTIVE INPUT

FILTER

COMPARE

. However, the

COMPARE

AND

FILTER

AND

FAULT

TO MULTIPLIER

is within

FAULT

TO MULTIPLIER

, the

04407-0-028

Calibration Concerns

Typically, when a meter is being calibrated, the voltage and current circuits are separated as shown in Figure 28. This means that current passes through only the phase or neutral circuit. Figure 28 shows current being passed through the phase circuit. This is the preferred option, because the ADE7761 starts billing on the input V nected to V

on power-up. The phase circuit CT is con-

in the diagram. Since there is no current in the

neutral circuit, the FAULT indicator comes on under these conditions. However, this does not affect the accuracy of the calibration and can be used as a means to test the functionality of the fault detection.

TEST

URRENT

PHASE

240V RMS

NEUTRAL

AGND

RA*

RB*

VR*

*RB + VR = RF

Figure 28. Fault Conditions for Inactive Input Greater than Active Input

If the neutral circuit is chosen for the current circuit in the arrangement shown in Figure 28, this may have implications for the calibration accuracy. The ADE7761 powers up with the V

input active as normal. However, because there is no current in the phase circuit, the signal on V be flagged and the active input to be swapped to V

is zero. This causes a fault to

(neutral).

The meter can be calibrated in this mode, but the phase and neutral CTs might differ slightly. Because under no-fault conditions all billing is carried out using the phase CT, the meter should be calibrated using the phase circuit. Of course, both phase and neutral circuits can be calibrated.

MISSING NEUTRAL MODE

The ADE7761 integrates a novel fault detection that warns and allows the ADE7761 to continue to bill in case a meter is connected to only one wire (see Figure 29). For correct operation of the ADE7761 in this mode, the V ADE7761 must be maintained within the specified range (5 V ± 5%). The missing neutral detection algorithm is designed to work over a line frequency of 45 Hz to 55 Hz.

pin of the

04407-0-030

6.25% OF INACTIVE INPUT

ACTIVE POINT – INACTIVE INPUT

04407-0-029

Figure 27. Fault Conditions for Inactive Input Greater than Active Input

Rev. A | Page 20 of 28

ADE7761

240V RMS

POWER

GENERATOR

LOAD

RA*

RB*

VR*

*RB + VR = RF

V1A

04407-0-031

Figure 29. Missing Neutral System Diagram

The ADE7761 detects a missing neutral condition by continuously monitoring the voltage channel input (V

2P–V2N

). The FAULT pin is held high when a missing neutral condition is detected. In this mode, the ADE7761 continues to bill the energy based on the signal level on the current channel (see Figure 30). The billing rate or frequency outputs can be adjusted by changing the dc level on the MISCAL pin.

MISCA

ADC

A > B

HPF

ADC

B > A B <> A

MISSING NEUTRAL

GAIN ADJUSMENT

ZERO

CROSSING

DETECTION

DIGITAL-

FREQUENCY

CONVERTERS

Figure 30. Energy Calculation in Missing Neutral Mode

LPF

TO-

F1 F2

04407-0-032

Important Note for Billing of Active Energy

The ADE7761 provides pulse outputs—CF, F1, and F2— intended to be used for the billing of active energy. Pulses are generated at these outputs in two different situations.

Case 1: When the analog input V

– V2N complies with the

conditions described in Figure 32, CF, F1, and F2 frequencies are proportional to active power and can be used to bill active energy.

Case 2: When the analog input V

– V2N does not comply with

the conditions described in Figure 32, the ADE7761 does not measure active energy, but a quantity proportional to kAh. This quantity is used to generate pulses on the same CF, F1, and F2. This situation is indicated when the FAULT pin is high.

Analog Devices Inc. cautions users of the ADE7761:

• Billing active energy in Case 1 is consistent with the

understanding of the quantity represented by pulses on CF, F1, and F2 outputs (watt-hour).

• Billing active energy while the ADE7761 is in Case 2 must

be decided knowing that the entity measured by the ADE7761 in this case is ampere-hour and not watt-hour. Users should be aware of this limitation and decide if the ADE7761 is appropriate for their application.

Missing Neutral Detection

The ADE7761 continuously monitors the voltage input and detects a missing neutral condition when the voltage input peak value is smaller than 9% of the analog full scale or when no zero crossings are detected on this input (see Figure 31).

FILTER AND THRESHOLD

AGND

ADC

MISSING NEUTRAL

|V2|

< 9% OF FULL SCALE

PEAK

2P–V2N

9% OF FS

Figure 31. Missing Neutral Detection

Rev. A | Page 21 of 28

V2P–V

FSFS

NO ZERO-CROSSING ON V2OR

V2P–V

04407-0-033

ADE7761

The ADE7761 leaves the missing neutral mode for normal operation when both conditions are no longer valid—voltage peak value greater than 9% of full scale and zero crossing on the voltage channel detected (see Figure 32).

GND

|V2|

PEAK

ZERO-CROSSING ON V2

+9% OF FS –9% OF FS

Figure 32. Return to Normal Mode after Missing Neutral Detection

ADC

> 9% OF FULL SCALE

AND

V2P–V

FILTER AND THRESHOLD

Missing Neutral Gain Calibration

When the ADE7761 is in missing neutral mode, the energy is billed based on the active current input signal level. The calibration of the frequency outputs in this mode can be done with the MISCAL analog input pin. In this mode, applying a dc voltage of 330 mV on MISCAL is equivalent to applying, in normal mode, a pure sine wave on the voltage input with a peak value of 330 mV. The MISCAL input can vary from 0 V to 660 mV (see the Analog Inputs section). When set to 0 V, the frequency outputs are close to zero. When set to 660 mV dc, the frequency outputs are twice that when MISCAL is at 330 mV dc. In other words, Equation 7 can be used in missing neutral mode by replacing V2

FrequencyFF

by MISCAL

rms

√2:

rms

REF

MISSING NEUTRAL

FMISCAL

2/70.5

××

−

(8)

04407-0-034

where:

, F2 Frequency is the output frequency on F1 and F2 (Hz).

is the differential rms voltage signal on Channel 1 (V).

rms

MISCAL

is the differential rms voltage signal on the MISCAL

rms

pin (V).

is the reference voltage (2.5 V ± 8%) (V).

REF

is one of four possible frequencies selected by using the

1-4

logic inputs S0 and S1 (see Table 5).

Example

In normal mode, ac voltages of ±330 mV peak are applied to V1 and V2, and then the expected output frequency on F1 and F2 is calculated as follows:

= 1.7 Hz, SCF = S1 = S0 = 0

1–4

V1 = rms of 330 mV peak ac = 0.33/√

V2 = rms of 330 mV peak ac = 0.33/√

= 2.5 V (nominal reference value)

REF

FrequencyFF

frequencyFFFrequencyCF

Hz7.133.033.070.5

××

5.222

=×−=

Hz084.0

Hz4.564

In missing neutral mode, ac voltage of ±330 mV peak is applied to V1, no signal is connected on V2, and 330 mV dc input is applied to MISCAL. With the ADE7761 in the same configuration as the previous example, the expected output frequencies on CF, F1, and F2 are

Hz7.12/33.033.070.5

FrequencyFF

frequencyFFFrequencyCF

×××

5.22 Hz4.564,

=×=

Hz084.0

Rev. A | Page 22 of 28

ADE7761

APPLICATIONS

INTERFACING TO A MICROCONTROLLER FOR ENERGY MEASUREMENT

The easiest way to interface the ADE7761 to a microcontroller is to use the CF high frequency output with the output frequency scaling set to 2048 × F1, F2. This is done by setting SCF = 0 and S0 = S1 = 1 (see Table 7). With full-scale ac signals on the analog inputs, the output frequency on CF is approximately 5.5 kHz. Figure 33 illustrates one scheme that could be used to digitize the output frequency and carry out the necessary averaging mentioned in the previous section.

AVERAGE

FREQUENCY

ADE7761

REVP*

FAULT**

*REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR

DIRECTION OF ENERGY FLOW IS NEEDED.

*FAULT MUST BE USED TO RECORD ENERGY IN FAULT CONDITION.

Figure 33. Interfacing the ADE7761 to an MCU

As shown, the frequency output CF is connected to an MCU counter or port, which counts the number of pulses in a given integration time, determined by an MCU internal timer. The average power, proportional to the average frequency, is given by

The energy consumed during an integration period is given by

FREQUENCY

RIPPLE

TIME

TimePowerAverageEnergy =×=×=

COUNTER

UP/DOWN

PowerActiveAverageFrequencyAverage ==

Counter

Tim

MCU

LOGIC

±10%

Counter

Time

04407-0-035

CounterTime

For the purpose of calibration, this integration time could be 10 s to 20 s in order to accumulate enough pulses to ensure correct averaging of the frequency. In normal operation, the integration time could be reduced to 1 s or 2 s depending, for example, on the required update rate of a display. With shorter integration times on the MCU, the amount of energy in each update may still have a small amount of ripple, even under steady load conditions. However, over a minute or more, the measured energy has no ripple.

SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION

As shown in Table 5, the user can select one of four frequencies. This frequency selection determines the maximum frequency on F1 and F2. These outputs are intended to be used to drive the energy register (electromechanical or other). Because only four different output frequencies can be selected, the available frequency selection has been optimized for a meter constant of 100 impulses/kWh with a maximum current of between 10 A and 120 A. Table 8 shows the output frequency for several maximum currents (I cases, the meter constant is 100 impulses/kWh.

Table 8. F1 and F2 Frequency at 100 Impulses/kWh

(A) F1 and F2 (Hz)

MAX

12.5 0.083 25 0.166 40 0.266 60 0.4 80 0.533 120 0.8

The F

frequencies allow complete coverage of this range of

1–4

output frequencies on F1 and F2. When designing an energy meter, the nominal design voltage on Channel 2 (voltage) should be set to half-scale to allow for calibration of the meter constant. The current channel should also be no more than halfscale when the meter sees maximum load, which accommodates overcurrent signals and signals with high crest factors. Table 9 shows the output frequency on F1 and F2 when both analog inputs are half-scale. The frequencies listed in Table 9 align well with those listed in Table 8 for maximum load.

) with a line voltage of 240 V. In all

MAX

Rev. A | Page 23 of 28

ADE7761

Table 9. F1 and F2 Frequency with Half-Scale AC Inputs

Frequency on F1 and F2, Ch 1 and Ch 2,

S0 S1 F

0 0 1.72 0.085 0 1 3.44 0.17 1 0 6.86 0.34 1 1 13.5 0.68

When selecting a suitable F frequency output at I of 100 impulses/kWh should be compared with Column 4 of Table 9. The frequency that is closest in Table 9 determines the best choice of frequency (F maximum current of 40 A is being designed, the output frequency on F1 and F2 with a meter constant of 100 impulses/kWh is 0.266 Hz at 40 A and 240 V (from Table 8). Looking at Table 9, the closest frequency to 0.266 Hz in Column 4 is 0.17 Hz. Therefore, F2 (3.4 Hz; see Table 5) is selected for this design.

Frequency Outputs

Figure 2 shows a timing diagram for the various frequency outputs. The high frequency CF output is intended to be used for communications and calibration purposes. CF produces a 90 ms wide, active high pulse (t tional to active power. The CF output frequencies are given in Table 7. As in the case of F1 and F2, if the period of CF (t below 180 ms, the CF pulse width is set to half the period. For example, if the CF frequency is 20 Hz, the CF pulse width is 25 ms.

1–4

Half-Scale AC Inputs (Hz)

frequency for a meter design, the

1–4

(maximum load) with a meter constant

MAX

). For example, if a meter with a

1-4

) at a frequency that is propor-

) falls

No-Load Threshold

The ADE7761 includes a no-load threshold and startup current feature that eliminates creep effects in the meter. The ADE7761 is designed to issue a minimum output frequency. Any load generating a frequency lower than this minimum frequency does not cause a pulse to be issued on F1, F2, or CF. The minimum output frequency is given as 0.0045% of the full-scale output frequency. (See Table 7 for maximum output frequencies for ac signals).

For example, an energy meter with a meter constant of 100 impulses/kWh on F1, F2 using SCF = 1, S1 = 0, and S0 = 1, the maximum output frequency at F1 or F2 is 0.68 Hz and 43.52 Hz on CF. The minimum output frequency at F1 or F2 is

0.0045% of 0.68 Hz or 3.06 × 10

–5

Hz. This is 1.96 × 10–3 Hz at

CF (64 × F1 Hz).

In this example, the no-load threshold is equivalent to 1.1 W of load or a startup current of 4.6 mA at 240 V. Compare this value to the IEC61036 specification, which states that the meter must start up with a load equal to or less than 0.4% I meter, 0.4% of I

is equivalent to 20 mA.

. For a 5 A (IB)

Note that the no-load threshold is not enabled when using the high CF frequency mode: SCF = 0, S1 = S0 = 1.

NEGATIVE POWER INFORMATION

The ADE7761 detects when the current and voltage channels have a phase shift greater than 90 wrong connection of the meter or generation of negative power. The REVP pin output goes active high when negative power is detected, and active low when positive power is detected. The REVP pin output changes state as a pulse is issued on CF.

°. This mechanism can detect

Rev. A | Page 24 of 28

ADE7761

OUTLINE DIMENSIONS

7.50

7.20

6.90

20 11

5.60

5.30

8.20

5.00

SEATING PLANE

(RS-20)

0.25

0.09

7.80

7.40

8° 4° 0°

0.95

0.75

0.55

2.00 MAX

0.05 MIN COPLANARITY

0.10

Figure 34. 20-Lead Shrink Small Outline Package [SSOP]

0.65

BSC

COMPLIANT TO JEDEC STANDARDS MO-150AE

1.85

1.75

1.65

0.38

0.22

Dimensions shown in millimeters

ORDERING GUIDE

Model Temperature Range Package Description Package Option

ADE7761ARS –40°C to +85°C Shrink Small Outline Package RS-20 ADE7761ARSRL –40°C to +85°C Shrink Small Outline Package RS-20 ADE7761ARS-REF Reference Board

Rev. A | Page 25 of 28

ADE7761

DISCLAIMER

The ADE7761 provides pulse outputs—CF, F1, and F2— intended to be used for the billing of active energy. Pulses are generated at these outputs in two different situations.

Case 1: When the analog input V

conditions described in Figure 32, CF, F1, and F2 frequencies are proportional to active power and can be used to bill active energy.

Case 2: When the analog input V

– V2N complies with the

– V2N does not comply with

Analog Devices Inc. cautions users of the ADE7761:

• Billing active energy in Case 1 is consistent with the

understanding of the quantity represented by pulses on CF, F1, and F2 outputs (watt-hour).

• Billing active energy while the ADE7761 is in Case 2 must

Rev. A | Page 26 of 28

ADE7761

NOTES

Rev. A | Page 27 of 28

ADE7761

NOTES

D04407–0–2/04(A)

Rev. A | Page 28 of 28

Datasheet ADE7761 Datasheet (Analog Devices)

Specifications and Main Features

Frequently Asked Questions

User Manual

FEATURES

FUNCTIONAL BLOCK DIAGRAM

GENERAL DESCRIPTION

SPECIFICATIONS

TIMING CHARACTERISTICS

ABSOLUTE MAXIMUM RATINGS

ESD CAUTION

TERMINOLOGY

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

TYPICAL PERFORMANCE CHARACTERISTICS

OPERATION

POWER SUPPLY MONITOR

ANALOG INPUTS

Channel V1 (Current Channel)

Channel V2 (Voltage Channel)

MISCAL Input

Typical Connection Diagrams

INTERNAL OSCILLATOR

ANALOG-TO-DIGITAL CONVERSION

Antialias Filter

ACTIVE POWER CALCULATION

Power Factor Considerations

Nonsinusoidal Voltage and Current

HPF and Offset Effects

DIGITAL-TO-FREQUENCY CONVERSION

TRANSFER FUNCTION

Frequency Outputs F1 and F2

Frequency Output CF

Example

FAULT DETECTION

Fault with Active Input Greater than Inactive Input

Fault with Inactive Input Greater than Active Input

Calibration Concerns

MISSING NEUTRAL MODE

Important Note for Billing of Active Energy

Missing Neutral Detection

Missing Neutral Gain Calibration

Example

APPLICATIONS

INTERFACING TO A MICROCONTROLLER FOR ENERGY MEASUREMENT

SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION

Frequency Outputs

No-Load Threshold

NEGATIVE POWER INFORMATION

OUTLINE DIMENSIONS

ORDERING GUIDE

DISCLAIMER