MAXQ3183-RAN+ datasheet(54/102 Pages) MAXIM

54 / 102 page

Low-Power, Multifunction, Polyphase AFE

with Harmonics and Tamper Detect

______________________________________________________________________________________

Theory of Operation

Analog Front-End Operation

Whenever the MAXQ3183 is in one of the active operat-

ing modes (Run Mode or LOWPM Mode), the analog

front-end operates continuously, scanning up to eight

scan slots depending on the selected front-end config-

uration. For each analog scan slot that is enabled, one

of the eight differential input pairs is measured.

The SCAN_IX and SCAN_VX (X = A/B/C), SCAN_IN,

and SCAN_TE registers contain the settings for each

slot, which include whether the slot is enabled and the

differential input pair to measure during that scan slot.

The logical mapping of the slots is fixed in following

order:

• Slot 0—Phase A Current (IA)

• Slot 1—Phase A Voltage (VA)

• Slot 2—Phase C Current (IC)

• Slot 3—Phase C Voltage (VC)

• Slot 4—Phase B Current (IB)

• Slot 5—Phase B Voltage (VB)

• Slot 6—Neutral Current (IN)—disabled by default

• Slot 7—Temperature Measurement—disabled by

default

The required time for each analog scan slot measure-

ment (tC) is determined by the MAXQ3183 system

clock frequency and the setting of the R_ADCRATE

hardware register, as shown below:

tC = 1/fCLK x (R_ADCRATE[8:0] + 1)

Using the default register settings (R_ADCRATE = 167h

= 359d), the time for each analog slot measurement

(tC) is 45μs when the MAXQ3183 is running at 8MHz.

Since there are eight analog scan slots in the measure-

ment frame, the total time for all measurements (tFR) is

tC x 8. Using the default settings with the MAXQ3183

running at 8MHz, the entire sequence of measurements

takes 360μs to complete, which, in turn, means that

360μs will elapse, for example, between one phase A

current measurement and the next.

Even if some of the analog measurement slots (such as

neutral current or temperature measurement) are

skipped by setting the DADCNV bit in that slot’s regis-

ter to 1, the time period for that slot will remain in the

frame, ensuring that the total frame time is always tC x

8, regardless of which individual slots are enabled or

disabled.

Digital Signal Processing (DSP)

Terminology

Establishing the precise definitions of some of the terms

used in this document will assist in understanding how

the DSP functions.

Sample Period: The amount of time required to mea-

sure a single data element; 45μs, by default.

ADC Frame Period: The amount of time required for

the ADC to sample all analog inputs; always equal to 8

sample periods. The inverse of this value is the

frame

rate; by default 2778 samples per second. This is the

rate at which any particular signal is sampled by the

MAXQ3183.

Line Cycle: The period of time from one positive-going

zero crossing on a voltage channel to the next positive-

going zero crossing. At 50Hz, this is nominally 20ms; at

60Hz, this is nominally 16.67ms.

Cycle Count: The number of line cycles contained in a

single DSP cycle. An integer, this is typically set to

some value greater than one to minimize the effect of

load variations that may not occur in every line cycle.

By default, this value is 16.

DSP Cycle: The period of time over which line parame-

ters are calculated. Energy and other parameters are

accumulated once per DSP cycle. One DSP cycle is

the time of a line cycle multiplied by the cycle count.

NS: This value represents the number of ADC frame

periods in a DSP cycle. This is a noninteger calculated

value. For example, if the cycle count is set to unity,

and the line frequency is exactly 50Hz, the NS value

would be 20ms/360μs = 55.6.

Digital Processing

As voltage and current samples are collected, the

MAXQ3183 performs a variety of digital filtering,

accumulation, and processing calculations to arrive at

meter-reading values (such as line frequency, RMS

voltage and current, and active and reactive power)

that can then be read by the master. The MAXQ3183

calculates and detects values and conditions including

the following:

• Zero-crossing detection

• Line frequency and line period calculation

• RMS voltage (phase A, phase B, phase C)

• RMS current (phase A, phase B, phase C, neutral

current)

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