ADE7978 datasheet(33/120 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932

Rev. 0 | Page 33 of 120

Current Waveform Gain Registers

There is a multiplier in the signal path of each phase and neutral

current. The current waveform can be changed by ±100% by

writing a corresponding twos complement number to the 24-bit

signed current waveform gain registers (AIGAIN, BIGAIN,

CIGAIN, and NIGAIN). For example, if 0x400000 is written to

these registers, the ADC output is scaled up by 50%. To scale

the output by −50%, write 0xC00000 to the registers. Equation 5

describes mathematically the function of the current waveform

gain registers.

Current Waveform =

(5)













Gain

Current

Contents

Output

ADC

Changing the contents of the AIGAIN, BIGAIN, CIGAIN, or

NIGAIN register affects all calculations based on the current

of the corresponding phase, including the active, reactive, and

apparent energy and the current rms calculations. In addition,

waveform samples scale accordingly.

The serial ports of the ADE7978 work with 32-, 16-, or 8-bit

words, whereas the DSP works with 28-bit words. The 24-bit

AIGAIN, BIGAIN, CIGAIN, and NIGAIN registers are sign

extended to 28 bits and padded with four 0s for transmission

as 32-bit registers (see Figure 44).

28 27

24 23

24-BIT NUMBER

0000

BITS[27:24] ARE

EQUAL TO BIT 23

BIT 23 IS A SIGN BIT

Figure 44. 24-Bit xIGAIN Register Transmitted as a 32-Bit Signed Word

The ADE7933/ADE7932 contain a temperature sensor that is

internally multiplexed with the second voltage measurement,

V2P (see the Second Voltage Channel and Temperature

Measurement section). The ADE7978 assumes that all shunts

used in the system have the same temperature coefficient. The

24-bit signed register TEMPCO contains the value of the

temperature coefficient.

Assume that the shunt resistance, R, varies linearly according

to the following formula:

R = R0 × [1 + ε × (T − T0)]

(6)

where:

R0 is the shunt resistance at the nominal temperature, T0.

ε is the temperature coefficient of the shunt.

T is the temperature of the shunt.

To compensate for the increase in resistance, the current wave-

form must be divided by 1 + ε × (T − T0). Because ε is a very

small number, this expression is equivalent to a multiplication

by 1 − ε × (T − T0). This multiplication is introduced in the

datapath signal of each phase and neutral current.

Current Waveform =

ADC Output × [1 − ε × (T − T0)]

(7)

The 24-bit signed ATEMP0, BTEMP0, CTEMP0, and NTEMP0

registers represent the ambient temperature (T0) at which the

meter temperature sensor gain calibration was executed on

every phase (see the Second Voltage Channel and Temperature

Measurement section). The 24-bit signed ATEMP, BTEMP,

CTEMP, and NTEMP registers represent the shunt tempera-

tures (T) measured by the temperature sensor of every ADE7933/

ADE7932 in the system.

The temperature sensor measurement starts when the VT_A,

VT_B, VT_C, and VT_N pins of the ADE7978 are set low; the

results are first stored in the ATEMP, BTEMP, CTEMP, and

NTEMP registers after 1.024 sec (see the Second Voltage Channel

and Temperature Measurement section). At this point, the temp-

erature compensation scheme becomes active and works at an

8 kHz update rate. Therefore, ATEMP, BTEMP, CTEMP, and

NTEMP represent the temperature (T) in Equation 7.

Equation 8 describes mathematically the function of the current

waveform temperature compensation.

Current Waveform =

(8)

ADC Output ×

























−













TEMP0

TEMP

TEMPCO

IGAIN

where TEMPCO, TEMP, and TEMP0 represent the contents

of the registers with the same name.

A simple approach to implement temperature compensation is

to use the temperature measurements without any gain correction

(see the Second Voltage Channel and Temperature Measurement

section). The xTEMP and xTEMP0 registers contain the temper-

ature sensor measurements. Set the 24-bit signed register

TEMPCO to the following value:

TEMPCO = ε × k × 246

(9)

where:

ε is the temperature coefficient of the shunt.

k = 8.72101 × 10−5 is the gain correction of the temperature

measurement.

For example, if ε = 50 ppm/°C,

TEMPCO = round(50 × 10−6 × 8.72101 × 10−5 × 246) =

306,843 = 0x4AE9B

The maximum value that can be written to the TEMPCO

temperature coefficient that can be compensated equal to

ppm/

1367

72101

−

MAX

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