MAXQ3183-RAN+ datasheet(65/102 Pages) MAXIM

65 / 102 page

Low-Power, Multifunction, Polyphase AFE

with Harmonics and Tamper Detect

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• Read the meter quantity again to verify the calibra-

tion.

Note that these steps can occur more than once for a

given signal type to verify readings at different signal

levels.

There are two methods to read the meter in the above

second step. The first is to read the raw register associ-

ated with the value under calibration, for example,

A.VRMS for the phase A voltage channel; A.IRMS for

the phase A current channel, and A.ACT for phase A

real power.

The second calibration method assigns a pulse output

to the value being calibrated and measures the pulse

period. In practical use, the method chosen depends

on the specific application and the available equip-

ment. For example, in some applications the voltage

and current are of no concern, but the energy accumu-

lation must be very accurate. For these applications,

meter calibration sets with built-in pulse measurement

facilities can make the most sense.

The calibration procedure involves the following gener-

al steps:

• Calibrate voltage for a given phase by applying a

known voltage and adjusting the voltage gain

(A.V_GAIN for phase A) until the RMS voltage

(A.VRMS for phase A) reads the applied voltage in

the designated units.

• Calibrate current by applying a known current and

adjusting the current gain (A.I_GAIN for phase A)

until the RMS current (A.IRMS for phase A) reads the

applied current in the designated units. If desired,

the current can be calibrated at two points (low

range and high range) for more accuracy.

• Once the current gain and voltage gain are calibrat-

ed, the power/energy should not require any addi-

tional adjustment for most situations. Although, a

separate power gain register is available for further

fine-tuning of the power/energy accuracy. One must

keep in mind that anytime voltage or current is recali-

brated, the power or energy accuracy is naturally

affected. So the power gain should be recalibrated to

achieve the desired accumulative effect of voltage,

current, and power gains.

• Calibrate the phase offset by applying a power factor

load and adjusting the phase angle offset according-

ly. If desired, the phase offset can be calibrated at

up to three points for more accuracy.

Once these elements are calibrated for each phase, all

other information (power factor, reactive power, appar-

ent power, etc.) is also properly calibrated. The

descriptions in the following sections deal specifically

with phase A, but the same procedure is followed with

phases B and C.

Calibrating Voltage

Ensure that there is no previous value in the gain regis-

ter, A.V_GAIN, by setting this register to 0x4000.

• Apply a known voltage with RMS value close to the

desired maximum operating voltage (and less than

VFS/√2).

• Read the A.VRMS register. Note the value.

• Convert the known value to meter units by dividing it

by MU_VOLT (= VFS/224).

• Divide the applied value (in meter unit) by the value

read from the MAXQ3183. The result should be a

value between 0 and 2. If the value falls outside of

this range, you have probably miscalculated VFS.

• Multiply the calculated value by 214. The result is the

gain value to be programmed into A.V_GAIN. Ensure

the most significant bit is 0.

When the gain value is programmed, wait for 2 to 3

seconds, reread the RMS value from A.VRMS. Check

that the measured value is correct by comparing

A.VRMS against the applied voltage in meter unit.

Voltage Calibration Example

Assumptions: VFS is 558.1V. The applied voltage is 240

VRMS.

• Convert the applied voltage to meter units. This cal-

culation gives 240 x 224/558.1 = 7,214,714 =

0x006E1679.

• Read the A.VRMS register. You read 0x0708029.

This is 7,372,841 decimal.

• Divide the applied voltage by the voltage read from

the meter. The result is 7,214,714/7,372,841 =

0.97855.

• Convert to integer by multiplying 214: 16,384 x

0.97855 = 16,033 = 0x3EA1. Write this value to the

A.V_GAIN register.

Calibrating Current

Ensure that there is no previous value in the gain regis-

ter, A.I_GAIN, by setting this register to 0x4000.

• Apply a known current with RMS value close to the

desired maximum operating current (and much lower

than IFS/√2).

• Read the A.IRMS register. Note the value.

• Convert the known value to meter units by dividing it

by MU_AMP (= IFS/224).

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