MAXQ3181-RAN+ datasheet(50/84 Pages) MAXIM

50 / 84 page

Low-Power, Active Energy, Polyphase AFE

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• Energy accumulation (including energy pulse output

function)

• Overvoltage detection

• Overcurrent detection

• Undervoltage detection

Per Sample Operations

On every ADC frame, the input samples are processed

as follows:

• The voltage and current samples are read. The cur-

rent sample is shifted to account for the gain applied

in the PGA. The phase- and gain-corrected samples

are passed to the next stage.

• Both the current and voltage signals are passed

through highpass filters (HPF) specified by the

HPF_C variable.

• The current and voltage signals are now split into

several components. The first of these components is

squared and accumulated to begin the RMS current

and voltage process. The second is processed and

accumulated to begin the real power calculation.

The result is a set of accumulated values that represent

squared voltage, squared current, and real (active)

energy for both the entire usable spectrum and as fil-

tered by the peak filter. The real energy at this point

does not yet represent real power; to obtain usable

power values further processing is required. Each of

these values is further processed at the end of each

DSP cycle.

Per DSP Cycle Operations

At the end of each DSP cycle, accumulated information

is available that is used to calculate all other opera-

tional results in the meter. DSP cycles track the line fre-

quency and have a duration of the number of cycles

specified in the CYCNT register. On each phase, the

time required for CYCNT cycles to complete is calculat-

ed and this value is used to update the duration of one

DSP cycle, specified in the NS register.

NS contains the number of ADC frame periods in a sin-

gle DSP cycle. Because line frequency varies slightly

from cycle to cycle, and because the ADC frame clock

is not synchronized to the line, the value of NS is not an

integer, and varies slightly from DSP cycle to DSP cycle.

Because the value of NS is so critical to accurate calcu-

lation of energy, ensuring that it is correct on every

cycle is essential. There are two ways to manage the

slight variation of NS from cycle to cycle: first, one

could simply replace the old value of NS with the newly

calculated value on each DSP cycle. This means that

NS (and every other value in the meter, since they

depend on NS) would have a significant amount of

uncertainty. A better method is to use each newly cal-

culated value of NS as an input to a filter. The

output of

the filter is then the value of NS that is actually used in

calculations. In the MAXQ3181, this filter is controlled

by the AVG_NS register.

A second problem with updating NS on every line

cycles is the fact that noise impulses that occur at near-

ly the same time as the zero crossing can shift the zero

CURRENT INPUT

ADC

HPF

VOLTAGE INPUT

HPF

GAIN SEL

I_GAIN

V_GAIN

ADC

Figure 9. Per Sample Operations

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