ADE7759 datasheet(23/32 Pages) AD

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REV. 0

ADE7759

–23–

isolated interface to external calibration equipment. Figure 38

illustrates the Energy-to-Frequency conversion in the ADE7759.

The Energy-to-Frequency conversion is accomplished by accumu-

lating the Active power signal in a 24-bit register. An output pulse

is generated when there is a zero to one transition on the MSB

(most significant bit) of the register. Under steady load conditions

the output frequency is proportional to the Active Power. The output

frequency at CF, with full-scale ac signals on Channel 1 and Chan-

nel 2 and CFDEN = 000h, CFNUM = 000h, and APGAIN = 000h,

is approximately 5.593 kHz. This can be calculated as follows:

With the Active Power Gain register set to 000h, the average

value of the instantaneous power signal (output of LPF2) is

CCCDh or 52,429 decimal. An output frequency is generated

on CF when the MSB in the energy to frequency register (24 bits)

toggles, i.e., when the register accumulates 2

23. This means the

23/CCCDh times (or 159.999 times). Since

the update rate is 4/CLKIN or 1.1175

µs, the time between

MSB toggles (CF pulses) is given as:

159 999

1 1175

1 78799 10

5592 86

(

)

–

×=

µss

Equation 8 gives an expression for the output frequency at the

Energy-to-Frequency (ETF) output with the contents of CFDEN

and CFNUM registers are both zero.

ETF Output Hz

Average LPF Output

CLKIN

()

(8)

This output frequency is easily scaled by a pair of Calibration

Frequency Divider registers (CFDEN[11:0] and CFNUM[11:0]).

These frequency scaling registers are 12-bit registers that can

scale the output frequency by 1 to 2

12. The output frequency is

given by the expression below.

CF Hz

ETF Output Hz

CFNUM

CFDEN

()

[: ]

=×

11 0

(9)

For example, if the CF output frequency is 5.59286 kHz while

the contents of CFNUM and CFDEN are zero, the CF output

frequency can be set to 25 Hz by writing 8 BDh (2237 in decimal) to

the CFDEN register and 00Ah (10 in decimal) to the CFNUM

only to scale down the frequency from the ETF output. Therefore,

the content of CFDEN should always be set no less than that of

the CFNUM register, i.e., the maximum output frequency from

CF pin will never exceed that of the ETF output. The power-up

default value for CFDEN is 3Fh and CFNUM is 0h.

The output frequency will have a slight ripple at a frequency equal

to twice the line frequency. This is due to imperfect filtering of

the instantaneous power signal to generate the Active Power

signal—see Active Power Calculation section. Equation 3 gives

an expression for the instantaneous power signal. This is filtered

by LPF2, which has a magnitude response given by Equation 10.

fHz

()

18 9

(10)

The Active Power signal (output of LPF2) can be rewritten as

fHz

()

cos

=−













()

8 9

(11)

where fl is the line frequency (e.g., 60 Hz)

From Equation 6

E t

VIt

()

sin

=−

()

















()

41 2

8 9

(12)

From Equation 12 it can be seen that there is a small ripple in the

energy calculation due to a sin(2

ωt) component. This is shown

graphically in Figure 39. The Active Energy calculation is shown

by the dashed straight line and is equal to V

× I × t. The sinusoidal

ripple in the Active Energy calculation is also shown. Since the

average value of a sinusoid is zero, this ripple will not contribute

to the energy calculation over time. However, the ripple can be

observed in the frequency output, especially at higher output

frequencies. The ripple will get larger as a percentage of the fre-

quency at larger loads and higher output frequencies. The reason

is that at higher output frequencies the integration or averaging

time in the Energy-to-Frequency conversion process is shorter.

As a consequence, some of the sinusoidal ripple is observable in

the frequency output. Choosing a lower output frequency at CF

for calibration can significantly reduce the ripple. Also averaging

the output frequency by using a longer gate time for the counter

will achieve the same results.

SIGN 26

2–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8

APOS [15:0]

LPF2

WAVEFORM [23:0]

ENERGY-TO-FREQUENCY

MSB

TRANSITION

ACTIVE POWER OFFSET

CALIBRATION

CFDEN [11:0]

CFNUM [11:0]

ACTIVE POWER

SIGNAL – P

Figure 38. Energy-to-Frequency Conversion

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