MAXQ3180-RAN+ datasheet(37/48 Pages) MAXIM

37 / 48 page

Low-Power, Multifunction, Polyphase AFE

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Filter coefficients (bpf_b0f and bpf_a1f) are signed 16-

bit values and can be configured by the master. By

default they are tuned to 50Hz frequency, provided that

all other parameters are also set to default values. The

coefficients of the filters should be updated periodically

by the master to track the actual line frequency that

may deviate from the preset value. It is usually enough

to update only one coefficient (bpf_a1) to change the

filter’s peak frequency slightly.

The same mechanism of BPF can be used for a special

feature—measuring individual harmonics. The BPF

coefficients can be set to values that pass only the Nth-

order harmonics on the voltage channel. Then, the

VRMS output produces the Nth harmonics voltage and

the power output produces the Nth harmonics power.

The current IRMS can then be calculated by the master

as IRMS = Power/VRMS.

Phase Angle Compensation

Phase angle compensation requires two coefficients—

cosine and sine of the phase angle PA. The phase

compensation angle PA is calculated based on the log-

arithm of the raw RMS-current values using 3rd-order

curve fitting:

PA x 216 = PA_0 + (PA_1 x X) + (PA_2 x X2) + (PA_3 x X3)

Where

Where brackets [ ] denote the integer and ln_off is the

hard-coded value 6100h.

Curve-fitting coefficients are usually obtained during the

calibration process and loaded into RAM registers upon

initialization. Once the phase angle PA has been calcu-

lated, the MAXQ3180 calculates SIN(PA) and COS(PA)

and performs phase compensation for active and reac-

tive energy. By default, all curve-fitting coefficients are

zeros: PA_0 = PA_1 = PA_2 = PA_3 = 0. The calibration

procedure is discussed in the

Typical Calibration

Procedure for MAXQ3180 section.

Apparent Power

Apparent power is then calculated. The method used is

selectable using the APPSEL (OPMODE.3) bit. If

APPSEL = 0 (the default mode), then

Pwr_S = sqrt (Pwr_A2 + Pwr_R2)

If APPSEL = 1, then

Pwr_S = VRMS x IRMS x NS,

and scaled by shifting in order to produce the same

magnitude as the square-root method (APPSEL = 0).

Linearity Correction

The MAXQ3180 applies linearity compensation to

active, reactive, and apparent energy based on

whether input current signal level is in high range or in

low range. The signal is considered high range if the

voltage across the analog input pins is greater than

approximately 48mVP-P; otherwise the signal is consid-

ered low range. For high range, offset correction is

applied to the apparent power value:

Pwr_S_corr = Pwr_S - Eoff_hi x K

For low range, both offset and gain correction factors

are applied to the apparent power value:

Pwr_S_corr = (1 + Gain_lo/216) x (Pwr_S - Eoff_lo x K)

The factor K accounts for possible voltage and line fre-

quency changes:

K = [NS/216] x [VRMS/216]/216

Separate linearity compensation coefficients (Eoff_hi,

Eoff_lo, Gain_lo) are used for each phase. Initial values of

those coefficients are loaded upon default initialization;

the master can then overwrite coefficients if necessary.

Once the corrected apparent power has been calculat-

ed, the MAXQ3180 applies proportional correction to

active and reactive power, and applies common power

gain to all power samples:

Pwr_A_corr = (1 + E_gain/216) x Pwr_A x

Pwr_S_corr/Pwr_S,

Pwr_R_corr = (1 + E_gain/216) x Pwr_R x

Pwr_S_corr/Pwr_S,

Pwr_S_corr = (1 + E_gain/216) x Pwr_S_corr

The calibration procedure is discussed in the

Typical

Calibration Procedure for MAXQ3180 section.

Energy Accumulation Start Delay

All filters have a certain settling time before accurate

energy readings can be accumulated. To avoid accu-

mulation of invalid data from filters that are still settling,

an energy accumulation timeout period can be set in

the ACC_TIMO register. Together with the accumula-

tion enable (ACC_EN) bit, this provides a means to

control the preaccumulation delay. After reset, ACC_EN

= 0 and no energy is accumulated. After the

ACC_TIMO scan slots have elapsed, the MAXQ3180

sets ACC_EN = 1 and energy accumulation begins.

No-Load Feature

To avoid meter creep, no energy accumulation should

take place when calculated apparent power is less

than a certain threshold. The NOLOAD register can be

xIn IRMS

In off

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MAXQ3180-RAN+ Datasheet(PDF) 37 Page - Maxim Integrated Products

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