ADE7978 datasheet(39/120 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932

Rev. 0 | Page 39 of 120

CHANGING THE PHASE VOLTAGE DATAPATH

The ADE7978 can direct one phase voltage input to the compu-

tational datapath of another phase. For example, the Phase A

voltage can be introduced into the Phase B computational data-

path, which means that all powers computed by the ADE7978

in Phase B are based on the Phase A voltage and the Phase B

current. Table 16 lists the settings of the VTOIA[1:0] bits and

the configured phase voltage directed to the Phase A computa-

tional datapath.

Table 16. VTOIA[1:0] Bit Settings (CONFIG Register,

Bits[9:8])

VTOIA[1:0] Bits

Voltage Directed to the Phase A

Computational Datapath

00 (default)

Phase A voltage

Phase B voltage

Phase C voltage

Phase A voltage

Table 17 lists the settings of the VTOIB[1:0] bits and the configured

phase voltage directed to the Phase B computational datapath.

Table 17. VTOIB[1:0] Bit Settings (CONFIG Register,

Bits[11:10])

VTOIB[1:0] Bits

Voltage Directed to the Phase B

Computational Datapath

00 (default)

Phase B voltage

Phase C voltage

Phase A voltage

Phase B voltage

Table 18 lists the settings of the VTOIC[1:0] bits and the con-

figured phase voltage directed to the Phase C computational

datapath.

Table 18. VTOIC[1:0] Bit Settings (CONFIG Register,

Bits[13:12])

VTOIC[1:0] Bits

Voltage Directed to the Phase C

Computational Datapath

00 (default)

Phase C voltage

Phase A voltage

Phase B voltage

Phase C voltage

Figure 50 shows the Phase A voltage used in the Phase B data-

path, the Phase B voltage used in the Phase C datapath, and the

Phase C voltage used in the Phase A datapath.

PHASE A

COMPUTATIONAL

DATAPATH

PHASE B

COMPUTATIONAL

DATAPATH

PHASE C

COMPUTATIONAL

DATAPATH

VTOIA[1:0] = 10,

PHASE C VOLTAGE

DIRECTED

TO PHASE A

VTOIB[1:0] = 10,

PHASE A VOLTAGE

DIRECTED

TO PHASE B

VTOIC[1:0] = 10,

PHASE B VOLTAGE

DIRECTED

TO PHASE C

CPHCAL

BPHCAL

APHCAL

Figure 50. Phase Voltages Used in Different Datapaths

REFERENCE CIRCUITS

The nominal reference voltage at the REF pin of the ADE7933/

ADE7932 is 1.2 V. This reference voltage is used for the ADCs.

Because the on chip dc-to-dc converter cannot supply external

loads, the REF pin cannot be overdriven by a standalone external

voltage reference.

The voltage of the ADE7933/ADE7932 reference drifts slightly

with temperature. Table 6 specifies the gain drift over temperature

for each ADC channel. The gain drift includes the temperature

variation of the ADC gain, together with the temperature varia-

tion of the internal voltage reference. The value of the gain

temperature drift varies from device to device.

Because the energy calculation uses two ADC channels, one for

the current and one for the voltage, any x% drift in the gain results

in a 2x% deviation of the meter accuracy. The reference drift

resulting from temperature changes is usually very small and is

typically much smaller than the drift of other components on a

meter. As an alternative, the meter can be calibrated at multiple

temperatures.

The VA2, VB2, VC2, and VN2 voltages and the temperature

sensor use the third ADC of the ADE7933/ADE7932, so any x%

drift in the gain results in an x% deviation of these measurements.

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