ADE7913 datasheet(20/44 Pages) AD

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ADE7912/ADE7913

Data Sheet

Rev. 0 | Page 20 of 44

ADC Transfer Function

All ADCs in the ADE7912/ADE7913 produce 24-bit signed

output codes. With a full-scale input signal of 31.25 mV on the

current channel and 0.5 V on the voltage channels, and with an

internal reference of 1.2 V, the ADC output code is nominally

5,320,000 and usually varies for each ADE7912/ADE7913

around this value. The code from the ADC can vary between

0x800000 (−8,388,608) and 0x7FFFFF (+8,388,607); this is

equivalent to an input signal level of ±49.27 mV on the current

channel and ±0.788 V on the voltage channels. However, for

specified performance, do not exceed the nominal range of

±31.25 mV for the current channel and ±500 mV for the voltage

channels; ADC performance is guaranteed only for input

signals within these limits.

ADC Output Values

The ADC output values are stored in three 24-bit signed

registers, IWV, V1WV, and V2WV, at a rate defined by Bits[5:4]

(ADC_FREQ) in the CONFIG register. The output frequency is

8 kHz (CLKIN/512), 4 kHz (CLKIN/1024), 2 kHz (CLKIN/2048),

or 1 kHz (CLKIN/4096) based on ADC_FREQ being equal to

00, 01, 10, or 11, respectively, when CLKIN is 4.096 MHz.

The microcontroller reads the ADC output registers one at a

time or in burst mode. See the SPI Read Operation and the SPI

Read Operation in Burst Mode sections for more information.

REFERENCE CIRCUIT

The nominal reference voltage at the REF pin is 1.2 V. This

reference voltage is used for the ADCs in the ADE7912/

ADE7913. 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 ADE7912/ADE7913 reference drifts slightly

with temperature. Table 1 lists the gain drift over temperature

specification of each ADC channel. This value includes the

temperature variation of the ADC gain, together with the

temperature variation of the internal voltage reference.

CRC OF ADC OUTPUT VALUES

Every output cycle, the ADE7912/ADE7913 compute the cyclic

redundancy check (CRC) of the ADC output values stored in

the IWV, V1WV, and V2WV registers. Bits[5:4] (ADC_FREQ)

in the CONFIG register determine the ADC output frequency

and, therefore, the update rate of the CRC. The CRC algorithm

is based on the CRC-16-CCITT algorithm. The registers are

introduced into a linear feedback shift register (LFSR) based

generator one byte at a time, least significant byte first, as shown in

Figure 28. Each byte is then used with the most significant bit first.

The 16-bit result is written in the ADC_CRC register.

Figure 28. CRC Calculation of ADC Output Values

Figure 29. LFSR Generator Used for ADC_CRC Calculation

Figure 29 shows how the LFSR works. The IWV, V1WV, and

V2WV registers form the [a71, a70,…, a0] bits used by the LFSR.

Bit a0 is Bit 7 of the first register to enter the LFSR; Bit a71 is

Bit 16 of V2WV, the last register to enter the LFSR. The

formulas that govern the LFSR are as follows:

bi(0) = 1, where i = 0, 1, 2, …, 15, the initial state of the bits that

form the CRC. Bit b0 is the least significant bit, and Bit b15 is the

most significant bit.

gi, where i = 0, 1, 2, …, 15 are the coefficients of the generating

polynomial defined by the CRC-16-CCITT algorithm as

follows:

G(x) = x16 + x12 + x5 + 1

(1)

g0 = g5 = g12 = 1

(2)

All other gi coefficients are equal to 0.

FB(j) = aj − 1 XOR b15(j − 1)

(3)

b0(j) = FB(j) AND g0

(4)

bi(j) = FB(j) AND gi XOR bi − 1(j − 1), i = 1, 2, 3, …, 15 (5)

Equation 3, Equation 4, and Equation 5 must be repeated for j =

1, 2, …, 72. The value written into the ADC_CRC register contains

Bit bi(72), i = 0, 1, …, 15.

The ADC_CRC register can be read by executing an SPI

operation. See the SPI Read Operation and the SPI Read

Operation in Burst Mode sections for more details.

TEMPERATURE SENSOR

The ADE7912/ADE7913 contain a temperature sensor that is

multiplexed with the V2P input of the voltage channel. Bit 3

(TEMP_EN) of the CONFIG register selects what the third

ADC of the ADE7913 measures. If the TEMP_EN bit is 0, its

default value, the ADC measures the voltage between the V2P

and VM pins. If the TEMP_EN bit is 1, the ADC measures the

temperature sensor. In the case of the ADE7912, the ADC always

measures the temperature sensor, and the state of the TEMP_EN

bit has no significance. In both the ADE7912 and the ADE7913,

the conversion result is stored in the V2WV register. The time it

LFSR

GENERATOR

a71

a48 a47

a24 a23

IWV REGISTER

V1WV REGISTER

V2WV REGISTER

LFSR

g15

b15

a71, a70,....,a2, a1, a0

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