ADE7755

Rev. A | Page 15 of 20

HPF and Offset Effects

Figure 29 shows the effect of offsets on the active power calculation.

An offset on Channel 1 and Channel 2 contributes a dc component

after multiplication. Because the dc component is extracted by

the LPF, it accumulates as active power. If not properly filtered, dc

offsets introduce error to the energy accumulation. This problem is

easily avoided by enabling the HPF (that is, the AC/DC pin is

set to logic high) in Channel 1. By removing the offset from at

least one channel, no error component can be generated at dc

by the multiplication. Error terms at cos(ωt) are removed by the

LPF and the digital-to-frequency conversion (see the

section).

Digital-to-

Frequency Conversion

)

2

cos(

2

)

cos(

)

cos(

2

t

I

V

t

V

I

t

I

V

I

V

I

V

OS

OS

OS

OS

ω

×

×

+

ω

×

+

ω

×

+

×

+

×

DC COMPONENT (INCLUDING ERROR TERM)

IS EXTRACTED BY THE LPF FOR ACTIVE

POWER CALCULATION

V × I

2

0

ω

2ω

FREQUENCY (RAD/s)

Figure 29. Effect of Channel Offset on the Active Power Calculation

The HPF in Channel 1 has an associated phase response that is

compensated for on chip. The phase compensation is activated

when the HPF is enabled and is disabled when the HPF is not

activated. Figure 30 and Figure 31 show the phase error between

channels with the compensation network activated. The ADE7755

is phase compensated up to 1 kHz, as shown. This ensures correct

active harmonic power calculation even at low power factors.

FREQUENCY (Hz)

0

100

200

300

400

500

600

700

800

900 1000

–0.05

–0.10

0

0.05

0.10

0.15

0.20

0.25

0.30

Figure 30. Phase Error Between Channels (0 Hz to 1 kHz)

FREQUENCY (Hz)

40

45

50

55

60

65

70

–0.05

–0.10

0

0.05

0.10

0.15

0.20

0.25

0.30

Figure 31. Phase Error Between Channels (40 Hz to 70 Hz)

DIGITAL-TO-FREQUENCY CONVERSION

The digital output of the low-pass filter after multiplication

contains the active power information. However, because this

LPF is not an ideal brick-wall filter implementation, the output

signal also contains attenuated components at the line frequency

and its harmonics, that is, cos(hωt) where h = 1, 2, 3, and so on.

The magnitude response of the filter is given by

)

Hz

8.9

/

(

1

1

)

(

f

f

H

+

=

(4)

For a line frequency of 50 Hz, the filter gives an attenuation

of the 2ω (100 Hz) component of approximately −22 dB. The

dominating harmonic is at twice the line frequency, that is,

cos(2 ωt), which is due to the instantaneous power signal.

Figure 32 shows the instantaneous active power signal at the

output of the LPF, which still contains a significant amount of

instantaneous power information, that is, cos(2 ωt). This signal

is then passed to the digital-to-frequency converter where it is

integrated (accumulated) over time to produce an output frequency.

This accumulation of the signal suppresses or averages out any

non-dc components in the instantaneous active power signal. The

average value of a sinusoidal signal is 0. Therefore, the frequency

generated by the ADE7755 is proportional to the average active

power. Figure 32 shows the digital-to-frequency conversion for

steady load conditions, that is, constant voltage and current.