ADE7759ARS datasheet(12/32 Pages) AD

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REV. 0

ADE7759

–12–

It is also possible to adjust offset errors on Channel 1 and Channel 2

by writing to the Offset Correction registers (CH1OS and CH2OS

respectively). These registers allow channel offsets in the range

±24 mV to ±50 mV (depending on the gain setting) to be removed.

Note that it is not necessary to perform an offset correction in an

energy measurement application if HPF1 Channel 1 is switched

on. Figure 6 shows the effect of offsets on the real power calcula-

tion. As seen in Figure 6, an offset on Channel 1 and Channel 2

will contribute a dc component after multiplication. Since this

dc component is extracted by LPF2 to generate the Active (Real)

Power information, the offsets will have contributed an error to

the Active Power calculation. This problem is easily avoided by

enabling HPF1 in Channel 1. By removing the offset from at

least one channel, no error component is generated at dc by the

multiplication. Error terms at cos(

ω t) are removed by LPF2 and

by integration of the Active Power signal in the Active Energy

DC COMPONENT (INCLUDING ERROR TERM)

IS EXTRACTED BY THE LPF FOR REAL

POWER CALCULATION

IOS

VOS

IOS

Figure 6. Effect of Channel Offsets on the Real

Power Calculation

The contents of the Offset Correction registers are 6-bit, sign,

and magnitude coded. The weighting of the LSB size depends

on the gain setting, i.e., 1, 2, 4, 8, or 16. Table II shows the

correctable offset span for each of the gain settings and the LSB

weight (mV) for the Offset Correction registers. The maximum

value that can be written to the offset correction registers is

±31

decimal—see Figure 7.

Table II. Offset Correction Range

Gain

Correctable Span

LSB Size

±50 mV

1.61 mV/LSB

±37 mV

1.19 mV/LSB

±30 mV

0.97 mV/LSB

±26 mV

0.84 mV/LSB

±24 mV

0.77 mV/LSB

Figure 7 shows the relationship between the Offset Correction

gain setting of one. In order to perform an offset adjustment, the

analog inputs should be first connected to AGND, and there

should be no signal on either Channel 1 or Channel 2. A read

from Channel 1 or Channel 2 using the Waveform register will

give an indication of the offset in the channel. This offset can be

canceled by writing an equal but opposite offset value to the

relevant offset register. The offset correction can be confirmed by

performing another read. Note that when adjusting the offset of

Channel 1, the digital integrator and the HPF1 should be disabled.

CH1OS[5:0]

SIGN + 5 BITS

+50mV

OFFSET

ADJUST

3Fh

00h

1Fh

–50mV

0mV

SIGN + 5 BITS

01,1111b

11,1111b

Figure 7. Channel Offset Correction Range (Gain = 1)

di/dt CURRENT SENSOR AND DIGITAL INTEGRATOR

di/dt sensor detects changes in magnetic field caused by ac

current. Figure 8 shows the principle of a di/dt current sensor.

MAGNETIC FIELD CREATED BY CURRENT

(DIRECTLY PROPORTIONAL TO CURRENT)

EMF (ELECTROMOTIVE FORCE)

INDUCED BY CHANGES IN

MAGNETIC FLUX DENSITY (di/dt)

–

Figure 8. Principle of a di/dt Current Sensor

The flux density of a magnetic field induced by a current is directly

proportional to the magnitude of the current. The changes in

the magnetic flux density passing through a conductor loop

generate an electromotive force (EMF) between the two ends of

the loop. The EMF is a voltage signal that is proportional to the

di/dt of the current. The voltage output from the di/dt current

sensor is determined by the mutual inductance between the

current-carrying conductor and the di/dt sensor. Figure 9 shows

the mutual inductance produces a di/dt signal at the output of

the sensor.

MUTUAL INDUCTANCE M

i(t)

–

v = M

di(t)

Figure 9. Mutual Inductance Between the di/dt

Sensor and the Current Carrying Conductor

The current signal needs to be recovered from the di/dt signal

before it can be used for active power calculation. An integrator

is therefore necessary to restore the signal to its original form.

The ADE7759 has a built-in digital integrator to recover the

current signal from the di/dt sensor. The digital integrator on

Channel 1 is switched on by default when the ADE7759 is

powered up. Setting the MSB of the CH1OS register to 0 will

turn off the integrator. Figures 10 to 13 show the magnitude and

phase response of the digital integrator.

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