ADE7759_15 datasheet(23/36 Pages) AD

23 / 36 page

REV. A

ADE7759

–23–

load conditions. This output frequency can provide a simple,

single-wire, optically isolated interface to external calibration

equipment. Figure 38 illustrates the energy-to-frequency con-

version in the ADE7759.

The energy-to-frequency conversion is accomplished by accu-

mulating 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 con-

ditions the output frequency is proportional to the active power.

The output frequency at CF, with full-scale ac signals on

Channel 1 and Channel 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

(

)

–

·=

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 deci-

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

CFNUM register. Note that the CFNUM and CFDEN regis-

ters are meant 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 filter-

ing 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:

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 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

frequency at larger loads and higher output frequencies. The rea-

son 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

ADE7759 Datasheet(PDF) 23 Page - Analog Devices

ADE7759 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL