MCP39F521 datasheet(38/52 Pages) MICROCHIP

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MCP39F521

DS20005442A-page 38

 2015 Microchip Technology Inc.

8.0

MCP39F521 CALIBRATION

8.1

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 MCP39F521 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.

8.2

Calibration Order

The proper steps for calibration need to be observed.

If the device has an external temperature sensor

attached, temperature calibration should be done first

by reading the value from the Thermistor Voltage

Temperature Reference Voltage register.

If the device runs on the internal oscillator, the line

frequency must be calibrated next using the

Auto-Calibration Frequency

command.

The single-point gain calibration at unity power factor

should be performed next.

If non-unity displacement power factor measurements

are a concern, then the next step should be phase

calibration, followed by reactive power gain calibration.

To summarize the order of calibration:

Temperature Calibration (optional)

Line Frequency Calibration (optional)

Gain Calibration at PF = 1

Phase Calibration at PF

 1 (optional)

Reactive Gain Calibration at PF

 1(optional)

8.3

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.

8.3.1

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

• Gain Voltage RMS

• Gain Active Power

When this command is issued, the MCP39F521

attempts to match the expected values to the

measured values for all three output quantities by

changing the gain register based on the following

formula:

EQUATION 8-1:

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 register) and the

previous gain must be such that the 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.

8.3.2

EXAMPLE OF RANGE SELECTION

FOR VALID CALIBRATION

In this example, the user applies a calibration current

of 1A to an uncalibrated system. The indicated value

in the Current RMS register is 2300 with the system's

specific shunt value, PGA gain, etc. The user expects

to see a value of 1000 in the Current RMS register

when 1A current is applied, meaning 1.000A with

1 mA resolution. Other given values are:

• The existing value for Gain Current RMS is 33480

• The existing value for Range is 12

GAIN

NEW

GAIN

OLD

Expected

Measured

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



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