REV. B

AD7755

–11–

Using Equations 1 and 2, the real power P can be expressed in

PP

=+

1

where:

PV

I

11

1

1

1

1

1

=×

=

cos

–

φ

φα

β

(3)

and

PVh

Ih

h

hh

h

H

h

=×

=

≠

∝

∑

1

cos

–

φ

φα

β

(4)

As can be seen from Equation 4 above, a harmonic real power

component is generated for every harmonic, provided that har-

monic is present in both the voltage and current waveforms.

The power factor calculation has previously been shown to be

accurate in the case of a pure sinusoid, therefore the harmonic

real power must also correctly account for power factor since it

is made up of a series of pure sinusoids.

Note that the input bandwidth of the analog inputs is 14 kHz

with a master clock frequency of 3.5795 MHz.

ANALOG INPUTS

Channel V1 (Current Channel )

The voltage output from the current transducer is connected to

the AD7755 here. Channel V1 is a fully differential voltage

input. V1P is the positive input with respect to V1N.

The maximum peak differential signal on Channel 1 should be

less than

±470 mV (330 mV rms for a pure sinusoidal signal) for

specified operation. Note that Channel 1 has a programmable

gain amplifier (PGA) with user selectable gain of 1, 2, 8 or 16

(see Table I). These gains facilitate easy transducer interfacing.

DIFFERENTIAL INPUT

470mV MAX PEAK

+470mV

AGND

V1

V1P

–470mV

COMMON-MODE

100mV MAX

V1N

V1

Figure 22. Maximum Signal Levels, Channel 1, Gain = 1

The diagram in Figure 22 illustrates the maximum signal levels

on V1P and V1N. The maximum differential voltage is

±470 mV

divided by the gain selection. The differential voltage signal on

the inputs must be referenced to a common mode, e.g. AGND.

The maximum common mode signal is

±100 mV as shown in

Figure 22.

Table I. Gain Selection for Channel 1

Maximum

G1

G0

Gain

Differential Signal

001

±470 mV

012

±235 mV

108

±60 mV

1116

±30 mV

Channel V2 (Voltage Channel )

The output of the line voltage transducer is connected to the

AD7755 at this analog input. Channel V2 is a fully differential

voltage input. The maximum peak differential signal on Chan-

nel 2 is

±660 mV. Figure 23 illustrates the maximum signal

levels that can be connected to the AD7755 Channel 2.

DIFFERENTIAL INPUT

660mV MAX PEAK

+660mV

AGND

V2

V2P

–660mV

COMMON-MODE

100mV MAX

V2N

V2

Figure 23. Maximum Signal Levels, Channel 2

Channel 2 must be driven from a common-mode voltage, i.e.,

the differential voltage signal on the input must be referenced to

a common mode (usually AGND). The analog inputs of the

AD7755 can be driven with common-mode voltages of up to

100 mV with respect to AGND. However best results are

achieved using a common mode equal to AGND.

Typical Connection Diagrams

Figure 24 shows a typical connection diagram for Channel V1.

A CT (current transformer) is the current transducer selected for

this example. Notice the common-mode voltage for Channel 1

is AGND and is derived by center tapping the burden resistor

to AGND. This provides the complementary analog input sig-

nals for V1P and V1N. The CT turns ratio and burden resistor

Rb are selected to give a peak differential voltage of

±470 mV/

Gain at maximum load.

V1P

AGND

470mV

GAIN

Rb

Rf

Rf

CT

NEUTRAL

PHASE

IP

V1N

Cf

Cf

Figure 24. Typical Connection for Channel 1