MAXQ3181-RAN+ datasheet(51/84 Pages) MAXIM

51 / 84 page

Low-Power, Active Energy, Polyphase AFE

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crossing, affecting the accuracy of the energy mea-

sured during the preceding period. For this reason, a

second register, REJ_NS contains a value that specifies

how far a particular sample can deviate from the aver-

age and still be considered valid. If the period of the

newly acquired DSP cycle differs from the previously

accumulated average value by more than REJ_NS ADC

frames, NS is not updated with the new period (but the

energy is still accumulated).

With this discussion in mind, the signal path for the vari-

ous reported parameters can be reviewed.

RMS Volts and RMS Amps: First, the squared voltage

accumulation is divided by NS. This accomplishes the

“mean” part of the “root-mean-square” calculation.

Then, the square root of the result is taken, producing

the raw RMS calculation value.

On the voltage channel, the signal is ready for gain

compensation to be applied. But on the current channel,

there is an additional twist: depending on the amplitude

of the current, there may be a gain factor pre-applied

before the raw sample is available. To compensate for

inaccuracy in the gain factor for the amplifier and for

noise seen in the channel at high gain settings, it may

be necessary to provide linearity compensation.

There are three registers that manage the linearization

of the current signal: the X.OFFS_HI (X = A/B/C) regis-

ter contains a signed value that is added to the raw

RMS current signal before further processing; the

X.OFFS_LO register contains a signed value that is

added to the raw RMS current signal when the current

signal is below a low current threshold (1/32 of the full

scale) value; and the X.GAIN_LO register contains a

gain adjustment that is applied to the current signal

when the current signal is below the threshold value.

The practical effect of this is to turn what may be a

somewhat nonlinear response curve for the current sen-

sor to a much more linear response by two-piece

approximation.

The “high current” calibration term X.OFFS_HI is used

so long as the instantaneous current exceeds the low-

current threshold at some instant during a DSP cycle. As

long as this threshold is crossed during a DSP cycle, the

value in X.OFFS_HI controls the offset current.

When the input stays below the low-current threshold

for one DSP cycle, the X.OFFS_LO and X.GAIN_LO are

applied. The low-current calibration terms (X.GAIN_LO

and X.OFFS_LO) remain in effect until the peak of input

current waveform exceeds 1/32 of full-scale current at

any time during a DSP cycle.

As a final step, both voltage and current are passed

through an averaging filter that provides smoothing for

the signals. The amount of filtering is given in AVG_C.

Energy: Current sensors and other external circuitry

components could introduce a phase distortion to the

current signal, and this phase distortion may not be

constant at all current values. Consequently, for the

most precise measurements, the phase between the

voltage and current signals must be compensated. In

the MAXQ3181, 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-

LINEARIZATION

AVERAGE

AVG_C

IRMS

OFFS_HI

GAIN_LO

OFFS_LO

RAW_I

AVERAGE

AVG_C

VRMS

RAW_V

Figure 10. Computation of RMS Values

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