Single-Phase Power-Measurement

14 _____________________________________________________________________________________

Calibration

Four parameters can be calibrated to optimize system

performance.

Conversion to Physical Units

The output registers are in “meter” units, and need to be

scaled with the input circuits to yield meaningful physical

values. Two conversion coefficients are needed: the volt-

age transducer ratio (VTR) and the current transducer

ratio (ITR), each specifying the ratio between the input

and output of the corresponding transducer. The VTR

represents the input voltage that would produce a 1V

signal on the VP or VN pin. The ITR represents the input

current that would produce a 1V signal on the IL pin.

For example, if the voltage-sensing circuit consists of a

749kI and 1kI resistor-divider, then VTR = 750(V/V).

If the current-sensing circuit is a 20mI shunt, then 50A

current would produce 1V signal on the IL pin, so ITR =

50(A/V).

The following equations convert “meter” units into physi-

cal units:

volts.

The current RMS correction is:

IRMS = I_OFFS + I_OV x VRMS + [(I_GAIN +IGV x

correction.

Voltage RMS correction is:

correction.

Active power correction is:

before correction.

Apparent power is computer from the corrected voltage

and current:

S = VRMS x IRMS

Reactive power is computer from corrected S and P:

2

2

Q

S

P

=

−

Table 4. Calibration Parameters

Figure 3. Calibration Circuit Example

MAXQ314

VP

IL

AGND

749kΩ

20mΩ

1kΩ

LINE

NEUTRAL

LOAD

REGISTER

DESCRIPTION

V_GAIN

Voltage Gain Factor. This factor affects the voltage RMS output and power output. The VRMS output is scaled

I_GAIN

Current Gain Factor. This factor affects the current RMS output and power output. The IRMS output is scaled by

PA

Phase-Angle Compensation

LPFC

Lowpass Filter Coefficient. This factor affects the lowpass filtering. It can be left unchanged for typical configura-

tions. It is defined as: