MAXQ3180-RAN+ datasheet(72/101 Pages) MAXIM

72 / 101 page

Low-Power, Multifunction, Polyphase AFE

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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

be controlled with arbitrary precision, and because

high gain implies increased noise, it may be necessary

to calibrate the MAXQ3180 to maintain linearity at the

lowest inputs.

There are two settings that manage low-current lineari-

ty: an offset setting, OFFS_LO; and a gain setting,

GAIN_LO. Setting the offset is simple. Ensure no cur-

rent is flowing in the current circuit. Read X.IRMS. To

calculate offset use following formula:

If LINFRM = 0:

If LINFRM = 1:

offs = -X.IRMS

Program the offs into the OFFS_LO register.

So, if the user reads 0x0113 from the X.IRMS register and

LINFRM = 1, program 0xFEED into the OFFS_LO register.

Setting the GAIN_LO register means applying a current

below the low-current threshold, reading the value from

the MAXQ3180, and adjusting the gain accordingly. Note

that, unlike offset, the low-end gain is

added to the overall

gain provided in the I_GAIN register.

Apply a known current with peak value less than the

low-current threshold. Ensure that there is no previous

value in the low-current gain register, A.GAIN_LO, by

setting this register to 0x4000. Read the A.IRMS regis-

ter (0x1CC). Note the value. Convert the known value to

meter units by multiplying the known value (in amperes)

by 224 and dividing by IFS. Divide the results of this cal-

culation by the value read from the MAXQ3180. The

result should be a value between 0 and 2. Convert the

integer by multiplying 214, and ensure MSB is zero. The

result is the gain value to be programmed into

A.GAIN_LO.

Calibrating Power/Energy Gain

Once voltage and current have been calibrated, the

energy and power calculation automatically reflects the

calibrated voltage and current. However the energy

gain factor (X.E_GAIN, X = A/B/C) can be further tuned

to achieve even more accurate power and energy

result if necessary. For example, if the voltage and cur-

rent calibration sources are not as accurate as the

power/energy calibration source, then the additional

gain calibration may be necessary. The following pro-

cedure for power/energy gain calibration is outlined for

phase A.

• Apply a precision unity power factor power (applied

value) that is close to the desired normal operating

point.

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

• Convert the applied value to meter units by dividing it

by MU_PWR.

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

read from the MAXQ3180. The result should be a

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

this range, IFS and/or VFS have probably been mis-

calculated.

• Multiply the calculated value by 214, and ensure the

MSB is zero. The result is the gain value to be pro-

grammed into A.E_GAIN.

• When the gain value is programmed, wait for 1 to 2

seconds, then reread the power value from PWRP.A.

Check that the measured value is correct by compar-

ing PWRP.A against the applied power in meter unit.

Multipoint Phase Offset Calibration

To perform the calibration at three current levels, note

the raw current value (X.IRMS) at each point. Label the

current values, from highest to lowest, I0, I1, and I2.

Program X.PA0, X.PA1, and X.PA2 with the phase offset

values calculated at I0, I1, and I2, respectively, as

described in the

Calibrating Phase Offset section.

Finally, program I1THR with the average of I0 and I1,

and program I2THR with the geometric average of I1

and I2. Now as the current changes the phase offset is

adjusted accordingly. See Figure 16.

offs

XIRMS

=−

I2THR

I1THR

PA2

PA1

PA0

INPUT CURRENT

Figure 16. Phase Offset vs. Input Current Calibration

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