MCP39F521 datasheet(39/52 Pages) MICROCHIP

39 / 52 page

 2015 Microchip Technology Inc.

DS20005442A-page 39

MCP39F521

By using Equation 8-2, the calculation for GainNEW

yields:

EQUATION 8-2:

When

using

the

Auto-Calibration Gain

command, the result would be a failed calibration or a

NAK returned form the MCP39F521, because the

resulting GainNEW is less than 25,000.

The solution is to use the Range register to bring the

measured value closer to the expected value, such

that a new gain value can be calculated within the

limits specified above.

The Range register specifies the number of right-bit

shifts (equivalent to divisions by 2) after the

multiplication with the Gain Current RMS register.

Refer to Section 5.0, Calculation Engine (CE)

Description

for information on the Range register.

Incrementing the Range register by 1 unit, an

additional right-bit shift or ÷2 is included in the

calculation. Increasing the current range from 12 to 13

yields the new measured Current RMS register value

of 2300/2 = 1150. The expected (1000) and measured

(1150) are much closer now, so the expected new gain

should be within the limits:

EQUATION 8-3:

The resulting new gain is within the limits and the

device successfully calibrates Current RMS and

returns an ACK.

It can be observed that the range can be set to 14 and

the resulting new gain will still be within limits

(GainNEW = 58226). However, since this gain value is

close to the limit of the 16-bit Gain register, variations

from system to system (component tolerances, etc.)

might create a scenario where the calibration is not

successful on some units and there would be a yield

issue. The best approach is to choose a range value

that places the new gain in the middle of the bounds of

the gain registers described above.

In a second example, when applying 1A, the user

expects an output of 1.0000A with 0.1 mA resolution.

The example is starting with the same initial values:

EQUATION 8-4:

The GainNEW is much larger than the 16-bit limit of

65535, so fewer right-bit shifts must be introduced to

get the measured value closer to the expected value.

The user needs to compute the number of bit shifts

that will give a value lower than 65535. To estimate

this number:

EQUATION 8-5:

2.2 rounds to the closest integer value of 2. The range

value changes to 12 – 2 = 10; there are 2 less right-bit

shifts.

The new measured value will be 2300 x 22 = 9200.

EQUATION 8-6:

The resulting new gain is within the limits and the

device successfully calibrates Current RMS and

returns an ACK.

8.4

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 8-7:

Convert this from degrees to the resolution

provided in Equation 8-8:

EQUATION 8-8:

GAIN

NEW

GAIN

OLD

Expected

Measured

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



33480

1000

2300

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



14556

25 000





GAIN

NEW

GAIN

OLD

Expected

Measured

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



33480

1000

1150

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



29113

25 000



29113

65535



GAIN

NEW

GAIN

OLD

Expected

Measured

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



33480

10000

2300

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



145565

65535



145565

65535

------------------2.2

GAIN

NEW

GAIN

OLD

Expected

Measured

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



33480

10000

9200

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



36391

25 000



36391

65535



MEAS

Value in PowerFactor Register

32768

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

ANGLE

MEAS 



MEAS

 180



---------



acos

ANGLE

EXP 



EXP

 180



---------



acos



ANGLE

MEAS

ANGLE

EXP

–

 40



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