MAXQ3180-RAN+ datasheet(65/101 Pages) MAXIM

65 / 101 page

Low-Power, Multifunction, Polyphase AFE

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Units Conversion Examples

The conversions from meter units to physical units are

illustrated with the simplified input circuits in Figures 13

and 14. The voltage input circuit is a voltage-divider.

Current input is through a current transfer with turn ratio

of 2000:1.

The voltage transducer ratio (VTR) = (R1 + R2)/R2 =

545, VFS = 558.1V.

The current transducer ratio (ITR) = CT_N/(2 x R) =

2000/(2 x 10) =100 (A/V), IFS = 102.4A.

The input circuits should be designed to avoid getting

too close to the ADC input full sale at the specified

maximum ratings. So for the above circuits, we would

specify the maximum input current = 70A (RMS) and

maximum voltage = 390V (RMS), to ensure that peak of

sinusoudal waveform never exceeds IFS or VFS.

Use the default ADC timing tFR = 320μs, we get the fol-

lowing meter unit to physical unit conversion coeffi-

cients (these coefficients are not part of the MAXQ3180

registers):

MU_AMP = IFS/224 = 6.1E-6 (A)

MU_VOLT = VFS/224 = 33.3E-6 (V)

MU_PWR = VFS x IFS/232 = 13.3E-6 (W)

MU_ENR = VFS x IFS x tFR/216 = 77.5E-9 (Wh)

For example, if we get 0x07654AF0 from reading

0x1CC register (phase A current RMS), the current

value it represents is

0x07654AF0 x MU_AMP = 47.33 (A)

For some low-end host microcontrollers, doing the

above math multiplication above could be difficult. For

this reason, the MAXQ3180 provides conversions for

some commonly needed parameters through the

VOLT_CC, AMP_CC, PWR_CC, and ENR_CC registers.

For example, if you want to display current in the reso-

lution of 1mA, without having to use a multiplication

operation to convert from the meter unit value

0x07654AF0, you would set AMP_CC to 0x0190, and

read from virtual register 0x831 (phase A RMS current).

The output would be 0xB8E45170. Dropping the lower

2 bytes (right shifting 16 bits) gives 0xB8E4, or 47332

decimal (47332mA).

AMP_CC is computed as follows:

AMP_CC = (IFS/224)/AMP_LSB = MU_AMP/AMP_LSB

AMP_LSB = 0.001/216 (A)

IFS = 102.4A

AMP_CC = (102.4/224)/(0.001/216) = 400d = 0x0190

Calibration Procedure

Calibration Overview

Calibration ensures that the recorded voltage, current,

energy, and power are in accordance with the design

criteria. Before creating a calibration regimen, establish

the fundamental units of the meter: the full-scale volt-

age and current. Then adjust the gain registers using

calculated calibration constants to produce the expect-

ed reading in the raw current, voltage, energy, and

power factor registers.

The calibration constants should be stored in non-

volatile memory by the host microcontroller. Upon any

reset or loss of power, the host microcontroller must

reload the MAXQ3180 with the constants.

Calibration always follows a set of fundamental steps:

• Apply a known signal (voltage/current/power) to the

meter.

• Read the meter.

• Calculate the correction factor based on the differ-

ence between the applied signal level and the meter

reading.

• Write the correction factor to the appropriate register.

MAXQ3180

I0P

VCOMM

IA (AC)

I0N

Figure 14. Sample Current Input Circuit

MAXQ3180

VOP

VA (AC)

544k

Figure 13. Sample Voltage Input Circuit

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