MAXQ3183-RAN+ datasheet(57/102 Pages) MAXIM

57 / 102 page

Low-Power, Multifunction, Polyphase AFE

with Harmonics and Tamper Detect

______________________________________________________________________________________

But current sensors and other external circuitry compo-

nents introduce a phase distortion to the current signal,

and this phase distortion may not be constant at all cur-

rent values. Consequently, for the most precise mea-

surements, the phase between the voltage and current

signals must be compensated. In the MAXQ3183, the

energy signals are compensated for phase offset by

performing a complex multiplication of the signal with

the contents of the appropriate phase offset register.

Determining which phase offset register is appropriate is

a matter of comparing the incoming RMS current for the

phase with the contents of the I1THR and I2THR regis-

ters. It is the responsibility of the administrative software

to ensure that I1THR is greater than or equal to I2THR. If

the raw RMS current is greater than or equal to the con-

tents of I1THR, then the angle expressed in PA0 is used

to compensate the phase angle. If the raw RMS current

is less than I2THR, then the angle expressed in PA2 is

used to compensate the phase angle. And if the raw

RMS current falls between I1THR and I2THR then PA1 is

used to compensate the phase angle. In this way, a

three-piece stepwise approximation of the phase

response of the current sensor is available.

To use a constant phase compensation, set I1THR and

I2THR to zero and insert the phase compensation value

into PA0.

The same processing can be performed to calculate

the reactive energy value. But reactive energy can be

calculated in another way: calculate apparent energy

by multiplying the raw RMS volts and raw RMS current,

square this value, then subtract the squared real

power. The square root of this value is the reactive

energy.

Similarly, apparent energy can be calculated in either

of two ways: either as the product of the raw RMS volts

and amps, or as the square root of the sum of the

squares of the real and reactive energy. Which of these

is selected depends on the value of the APPSEL bit in

the OPMODE2 register: if 0, then apparent energy is

the product of the raw RMS volts and amps and reac-

tive energy is calculated using the difference of

squares method; if 1, apparent energy is calculated

using the sum of squares method and reactive energy

is calculated directly from the complex energy.

Line Frequency and Phasor Angles: Line frequency

can be taken directly from the NS value. Recall that NS

is the number of frames in a DSP cycle. Since each

frame is 360μs, simply multiply NS by 360μs and divide

by CYCNT to obtain the line period. The reciprocal of

this is the line frequency.

To calculate phasor angles, the numbers of samples

between zero crossings on phase A and B and on phase

A and C are taken. Since NS is the number of samples

during a complete DSP cycle, it is easy to calculate the

fraction of a complete cycle. The software then converts

this value to degrees and adjusts it such that no negative

angles are reported. No calibration is required for line

frequency and phasor angle calculation.

Energy Accumulation

Once real and reactive energy over the most recent

DSP cycle has been calculated, it is necessary to accu-

mulate the result.

For reactive energy, the result accumulated during any

DSP cycle may be positive (for an inductive load) or

I THR

PA I THR

I THR

RMS

≥

>≥

,,I

I THR

RMS <

⎧

⎨

⎪

⎩

⎪

⎫

⎬

⎪

⎭

⎪

PHASE

COMPENSATION

PA0

PA1

PA2

LINEARIZATION

AVERAGE

AVG_C

EREAL

OFFS_HI

GAIN_LO

OFFS_LO

LINEARIZATION

OFFS_HI

GAIN_LO

OFFS_LO

E_GAIN

E_RAWREAL

E_RAWREACTIVE

Figure 11. Phase Compensation for Energy Calculations

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