MAXQ3181-RAN+ datasheet(63/84 Pages) MAXIM

63 / 84 page

Low-Power, Active Energy, Polyphase AFE

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The filter coefficient is a signed 16-bit value and can be

configured by master. Here Y denotes the global NS

value, X denotes individual NS measurements pro-

duced by zero-crossing events detected on the phase

A, B, or C voltage channel. Note that if all three phase

voltages present, the filter above receives three inputs

each DSP cycle. The global NS value is used to gener-

ate the trigger for DSP processing. Note that the NS

value can be configured by the master, which could be

necessary if all three voltage signals are lost and no

zero-crossings are detected. The line period is then

calculated as a product of NS and the scan frame tFR.

The reciprocal of this value is the line frequency, which

can be obtained as a fixed-point value with 1 LSB =

0.001Hz by reading the LINEFR register.

Low-Power Measurement Mode (LOWPM)

This mode enables a subset of metering functions while

operating from the lower frequency internal RC oscilla-

tor to conserve power. The actual system clock fre-

quency used is the RC oscillator output frequency

divided by 8, which results in a system clock frequency

of approximately 1MHz.

The parameters provided in the LOWPM are:

• Voltage RMS

• Current RMS

• Ampere-Hour

The ampere-hour value is readable from the X.ESF reg-

isters (X = A/B/C). Entry to LOWPM mode only occurs

at the request of the master. The master must set the

LOWPM_E bit (register address 0xC03) to 1 to place

the MAXQ3181 into LOWPM mode. Entering LOWPM

mode changes the clock frequency, thereby invalidat-

ing a number of configuration registers. As a result, the

master must immediately reload the configuration regis-

ters and filter with new, updated values before metering

measurement operations can continue.

The master instructs the MAXQ3181 to exit LOWPM

mode by reading the LOWPM_X bit (register address

0xC04).

Temperature

The MAXQ3181 contains a temperature sensor that can

be used by host software for any purpose, including

compensating power readings for temperature effects.

Use the virtual register command (RAWTEMP, 0xC01)

to perform a temperature conversion. The MAXQ3181

returns raw ADC reading of voltage produced by the

temperature sensor.

Conversion from the arbitrary units to useful units (such

as degrees Celsius) requires taking one calibration

point and storing a conversion constant in the host

processor. The conversion constant is simply the value

(in absolute degrees) of one LSB.

To calculate the LSB value, take a reading at a known

temperature and divide the known temperature by the

reading. For example, assume you take a reading at

room temperature (23°C), and the reading is 0x7F00.

The degrees per LSB are then:

(23 + 273.15)/0x7F00 = 0.00911K

Now, assume at a later time you read the temperature

and see it is 0x84F0. To find the temperature in Celsius,

multiply by the degrees per LSB and subtract 273.15:

0x84F0 x 0.00911 - 273.15 = 36.8

°C

Advanced Calibrations

Calibrating Current Offset

Ideal hardware should produce a current reading lin-

early proportional to the input current. However, due to

noise or other factors, the RMS current read by the

meter might not be precisely linear. The current offset

(X.OFFS_HI, X = A/B/C) can be used to compensate

the current channel nonlinearity.

Since the MAXQ3181 tracks the input current to deter-

mine what linearity compensation factors to use, the

user must choose two points (ilo and ihi) comfortably

above the low current threshold, and get the X.IRMS

current readings (rlo and rhi). Then calculate the Y-inter-

cept of the line drawn between the two points, that is,

the offset. To calculate the value for the offset register,

use the following formula. If LINFRM = 0:

If LINFRM = 1:

In this equation, ihi and rhi are the applied current and

the current reading, respectively, in meter units at the

higher of the two reference currents; ilo and rlo are the

applied current and the current reading, respectively, in

meter units at the lower of the two reference currents.

The gain (X.I_GAIN) may require recalibration after the

offset register updated.

Calibrating Linearity

The current channel includes a variable-gain amplifier

that introduces a gain of 32 when the current falls

below the low current threshold (about 1/32 of full-scale

current IFS). Because the gain of the amplifier cannot

offs

i r

hi lo

−

()

2 4

offs

hi lo

−

)

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