MCP39F511N datasheet(45/61 Pages) MICROCHIP

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DS20005473B-page 45

MCP39F511N

9.0

MCP39F511N CALIBRATION

9.1

Overview

Calibration

compensates

for ADC

gain

error,

component tolerances and overall noise in the system.

The device provides an on-chip calibration algorithm

that allows simple system calibration to be performed

quickly. The excellent analog performance of the A/D

converters on the MCP39F511N allows for a

single-point calibration and a single calibration

command to achieve accurate measurements.

Calibration can be done by either using the predefined

auto-calibration commands, or by writing directly to the

calibration registers. If additional calibration points are

required (AC offset, Phase Compensation, DC offset),

the corresponding calibration registers are available to

the user and will be described separately in this

section.

9.2

Calibration Order

The proper steps for calibration need to be maintained.

If the device runs on the internal oscillator, the line

frequency should be calibrated first using the

Auto-Calibrate Frequency

command.

The single-point Gain Calibration at Unity Power Factor

should be performed next. This can be done for an

individual channel or for both channels at the same

time, depending on the user’s calibration setup.

If non-unity displacement power factor measurements

are a concern, then the next step should be Phase

calibration followed by Reactive power gain calibration.

Here is a summary on the order of calibration steps:

Line Frequency Calibration

Gain Calibration at PF = 1 for a single channel

or both

Phase Calibration at PF

 1 for a single channel

or both (optional)

Reactive Gain Calibration at PF

 1 for a single

channel or both (optional)

If calibrating a single channel at a time, repeat steps

2-4 for the second channel.

9.3

Single-Point Gain Calibrations at

Unity Power Factor

When using the device in AC mode with the high-pass

filters turned on, most offset errors are removed and

only a single-point gain calibration is required.

Setting the gain registers to properly produce the

desired outputs can be done manually by writing to the

appropriate register. The alternative method is to use

the auto-calibration commands described in this

section.

9.3.1

USING THE AUTO-CALIBRATION

GAIN COMMAND

By applying stable reference voltages and currents that

are equivalent to the values that reside in the target

Calibration Current, Calibration Voltage and Calibration

Active Power registers, the Auto-Calibration

Gain

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

registers

are

set

when

the

Auto-Calibration Gain

command is issued:

• Gain Current RMS Channel 1

• Gain Current RMS Channel 2

• Gain Voltage RMS

• Gain Active Power Channel 1

• Gain Active Power Channel 2

The channels can be calibrated individually or simulta-

neously depending on the instruction parameter byte

following the command byte.

When this command is issued, the MCP39F511N

attempts to match the expected values to the mea-

sured values for all three output quantities by changing

the gain register based on the following formula:

EQUATION 9-1:

The same formula applies for Voltage RMS,

Current RMS and Active power. Since the gain

registers for all three quantities are 16-bit numbers,

the ratio of the expected value to the measured value

(which can be modified by changing the Range

equation yields a valid number. Here the limits are set

to be from 25,000 to 65,535. A new gain within this

range for all three limits will return an ACK for a

successful calibration, otherwise the command returns

a NAK for a failed calibration attempt.

It is the user’s responsibility to ensure that the proper

range settings, PGA settings and hardware design

settings are correct to allow for successful calibration

using this command.

GAIN

NEW

GAIN

OLD

Expected

Measured

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



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