MAXQ3181-RAN+ datasheet(60/84 Pages) MAXIM

60 / 84 page

Low-Power, Active Energy, Polyphase AFE

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• Divide the applied value (in meter unit) by the value

read from the MAXQ3181. The result should be a

value between 0 and 2. If the value falls outside of

this range, you have probably miscalculated IFS.

• Multiply the calculated value by 214. The result is the

gain value to be programmed into A.I_GAIN. Ensure

the most significant bit is 0.

When the gain value is programmed, wait for approxi-

mately 2 to 3 seconds, then reread the RMS value from

A.IRMS. Check that the measured value is correct by

comparing A.IRMS against the applied current in meter

unit.

Calibrating Phase Offset

Phase offset calibration should be performed after the

voltage and current gains have been calibrated. To cal-

ibrate the phase offset, it is first necessary to measure

the power reported by the meter at two different phase

points. The best way to do this is to rely on the pulse

output of the meter; use a precise counter to determine

the power reported by the meter by counting the pulse

period. A load of power factor 0.5L or 0.5C is a good

choice for calibrating phase offset.

1) Apply a known pure resistive load to the meter.

Read the measured power, as P1.0.

2) Shift the phase of the current by +60° (power factor

of 0.5C). Read the measured power, as P0.5C.

3) The phase offset PA0.5C is computed from the

equation:

tan(PA0.5C) = (1 - 2P0.5C/P1.0)/

If an inductive load (PF = 0.5L) is applied, the

phase offset equation becomes:

tan(PA0.5L) = (2P0.5L/P1.0 - 1)/

These equations can be expressed in terms of rela-

tive errors, where E1.0, E0.5C, and E0.5L are the

power relative errors at PF = 1.0, 0.5C, and 0.5L

respectively.

4) Solve for PA from one of the above equations.

5) Convert PA into integer number, by multiplying it by

216.

6) Convert to hex value and write to the appropriate

Note that MAXQ3181 supports three offset values—

X.PA2, X.PA1, and X.PA0—corresponding to the low,

mid, and high range of the input signals, respectively.

Calibrating at the three loading levels could become

necessary if the phase error introduced by the current

sensors varies significantly with the input levels.

Registers I1THR and I2THR define the limits of the

ranges. If I1THR and I2THR are left at their default val-

ues of 0x0000, X.PA0 is applied to the full input range.

Phase Offset Calibration Example

Make sure X.PA0, X.PA1, and X.PA2 are cleared before

proceeding with calibrations. Assume the phase A has

been calibrated for A.VGAIN and A.IGAIN at Ib (= 10A).

At PF = 1.0, the active power relative error is E1.0 =

-0.4%. Set input to I = Ib and PF = 0.5L. The relative

error reported by the meter tester is E0.5L = 1.2%. Solve

for PA0.5L from the following equation:

where PA0.5L = 0.009274437 (radians) = 0.53° and

X.PA0 = 0.009274437 x 216 = 607.8095 = 0x0260.

Write 0x260 to A.PA0 (address 0x13E). If multirange

calibration is required, repeat the above procedure at

the desired input levels. The input levels for calibration

should be selected based the phase error characteris-

tics of the current sensors.

Interfacing the MAXQ3181 to

External Hardware

The MAXQ3181 has all the internal circuitry that is

needed for a sophisticated electricity meter, but specif-

ic external hardware is required when configuring the

meter for a particular application. The most critical

decision that must be made is how the load will be con-

nected to the power source, and how the meter will be

connected to measure power consumed in the load.

This section covers how to select hardware compo-

nents for a MAXQ3181 electricity meter.

tan(

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0 4 100 1

−+

+ 22 100

1 0 4 100

0 009274703

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−

tan(

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LAB

METER

UNIT

UNDER

TEST

LOAD

LINE

NEUTRAL

Figure 15. Offset Testing Setup

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