ADE7758_15 datasheet(27/72 Pages) AD

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Data Sheet

ADE7758

Rev. E | Page 27 of 72

On the ADE7758, if the regular succession of the zero crossings

presented above happens, the SEQERR bit (Bit 19) in the

STATUS register is set (Figure 60). If SEQERR is set in the mask

section).

Interrupts

If the regular zero crossing succession does not occur, that is when

a negative to positive transition on Phase A followed by a

positive to negative transition on Phase B, followed by a

negative to positive transition on Phase C, and so on, the

SEQERR bit (Bit 19) in the STATUS register is cleared to 0.

To have the ADE7758 trigger SEQERR status bit when the zero

crossing regular succession does not occur, the analog inputs for

Phase C and Phase B should be swapped. In this case, the Phase

B voltage input should be wired to the VCP pin, and the Phase

C voltage input should be wired to the VBP pin.

SEQERR BIT OF STATUS REGISTER IS SET

A = 0°

B = –120°

C = +120°

VOLTAGE

WAVEFORMS

ZERO

CROSSINGS

Figure 60. Regular Phase Sequence Sets SEQERR Bit to 1

SEQERR BIT OF STATUS REGISTER IS NOT SET

A = 0°

C = –120°

B = +120°

ZERO

CROSSINGS

VOLTAGE

WAVEFORMS

Figure 61. Erroneous Phase Sequence Clears SEQERR Bit to 0

POWER-SUPPLY MONITOR

The ADE7758 also contains an on-chip power-supply monitor.

The analog supply (AVDD) is monitored continuously by the

ADE7758. If the supply is less than 4 V ± 5%, the ADE7758

goes into an inactive state, that is, no energy is accumulated

when the supply voltage is below 4 V. 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. When AVDD returns above 4 V ± 5%, the

ADE7758 waits 18 μs for the voltage to achieve the

recommended voltage range, 5 V ± 5% and then becomes ready

to function. Figure 62 shows the behavior of the ADE7758

when the voltage of AVDD falls below the power-supply

monitor threshold. The power supply and decoupling for the

part should be designed such that the ripple at AVDD does not

exceed 5 V ± 5% as specified for normal operation.

AVDD

ADE7758

INTERNAL

CALCULATIONS

ACTIVE

INACTIVE

TIME

Figure 62. On-Chip, Power-Supply Monitoring

REFERENCE CIRCUIT

The nominal reference voltage at the REFIN/OUT pin is 2.42 V.

This is the reference voltage used for the ADCs in the

ADE7758. However, the current channels have three input

range selections (full scale is selectable among 0.5 V, 0.25 V, and

0.125 V). This is achieved by dividing the reference internally

by 1, ½, and ¼. The reference value is used for the ADC in the

current channels. Note that the full-scale selection is only

available for the current inputs.

The REFIN/OUT pin can be overdriven by an external source, for

example, an external 2.5 V reference. Note that the nominal

reference value supplied to the ADC is now 2.5 V and not

2.42 V. This has the effect of increasing the nominal analog

input signal range by 2.5/2.42 × 100% = 3% or from 0.5 V to

0.5165 V.

The voltage of the ADE7758 reference drifts slightly with

temperature; see the Specifications section for the temperature

coefficient specification (in ppm/°C). The value of the temperature

drift varies from part to part. Because the reference is used for

all ADCs, any ×% drift in the reference results in a 2×%

deviation of the meter accuracy. The reference drift resulting

from temperature changes is usually very small and typically

much smaller than the drift of other components on a meter.

Alternatively, the meter can be calibrated at multiple temperatures.

TEMPERATURE MEASUREMENT

The ADE7758 also includes an on-chip temperature sensor. A

temperature measurement is made every 4/CLKIN seconds.

The output from the temperature sensing circuit is connected to

an ADC for digitizing. The resultant code is processed and

placed in the temperature register (TEMP[7:0]). This register

can be read by the user and has an address of 0x11 (see the

Serial Interface section). The contents of the temperature

3°C/LSB. The offset of this register may vary significantly from

part to part. To calibrate this register, the nominal value should

be measured, and the equation should be adjusted accordingly.

ADE7758 Datasheet(PDF) 27 Page - Analog Devices

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