MCP39F511 datasheet(49/62 Pages) MICROCHIP

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DS20005393B-page 49

MCP39F511

9.4

Calibrating the Phase

Compensation Register

Phase compensation is provided to adjust for any

phase delay between the current and voltage path.

This procedure requires sinusoidal current and voltage

waveforms, with a significant phase shift between

them, and significant amplitudes. The recommended

displacement power factor for calibration is 0.5. The

procedure for calculating the phase compensation

Determine what the difference is between the

angle corresponding to the measured power

factor (PFMEAS) and the angle corresponding to

the expected power factor (PFEXP), in degrees.

EQUATION 9-7:

Convert this from degrees to the resolution

provided in Equation 9-8:

EQUATION 9-8:

Combine this additional phase compensation to

whatever value is currently in the phase

compensation,

and

update

the

Equation 9-9 should be computed in terms of an

8-bit 2's complement-signed value. The 8-bit

result is placed in the least significant byte of the

16-bit Phase Compensation register.

EQUATION 9-9:

Based on Equation 9-9, the maximum angle in degrees

that can be compensated is approximately ±3.2

degrees. If a larger phase shift is required, contact your

local Microchip sales office.

9.5

Offset/No-Load Calibrations

During offset calibrations, it is recommended that no

line voltage or current be applied to the system. The

system should be in a no-load condition.

9.5.1

AC OFFSET CALIBRATION

There are three registers associated with the AC Offset

Calibration:

• Offset Current RMS

• Offset Active Power

• Offset Reactive Power

When computing the AC offset values, the respective

gain and range registers should be taken into

consideration according to the block diagrams in

Figures 5-2 and 5-4.

After

successful

offset

calibration,

Save Registers to Flash

command can then be

issued to save the calibration constants calculated by

the device.

9.5.2

DC OFFSET CALIBRATION

In DC applications, the high-pass filter on the current

and voltage channels is turned off. To remove any

residual DC value on the current, the DCOffsetCurrent

the ADC and prior to any other function.

9.6

Calibrating the Line Frequency

The Line Frequency register contains a 16-bit number

with a value equivalent to the input-line frequency as it

is measured on the voltage channel. When in

DC mode, this calculation is turned off and the register

will be equal to zero.

The measurement of the line frequency is only valid

from 45 to 65 Hz.

9.6.1

USING THE AUTO-CALIBRATE

FREQUENCY COMMAND

By applying a stable reference voltage with a constant

line frequency that is equivalent to the value that

resides

the

Line

Frequency

Ref,

the

Auto-Calibrate Frequency

command can then

be issued to the device.

After a successful calibration (response = ACK), a

Save Registers to Flash

command can then be

issued to save the calibration constants calculated by

the device.

The

following

set

when

the

Auto-Calibrate Frequency

command is issued:

• Gain Line Frequency

Note that the command is only required when running

off the internal oscillator. The formula used to calculate

the new gain is shown in Equation 9-1.

MEAS

Value in PowerFactor Register

32768

---------------------------------------------------------------------------

ANGLE

MEAS 



MEAS

 180



---------



acos

ANGLE

EXP 



EXP

 180



---------



acos



ANGLE

MEAS

ANGLE

EXP

–

 40



PhaseCompensation

NEW

PhaseCompens ation

OLD



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