MAXQ3183-RAN+ datasheet(69/102 Pages) MAXIM

69 / 102 page

Low-Power, Multifunction, Polyphase AFE

with Harmonics and Tamper Detect

______________________________________________________________________________________

• R_ADCACQ: Modify this register to change the

acquisition time. The acquisition time is the time from

ADC power-on until conversion starts, and is provid-

ed to allow the input amplifiers to settle. By default

this is set to 47 decimal, or 6μs at an 8MHz system

clock. If the system clock rate is changed, then

R_ADCACQ should change so that this value

remains about 6μs.

Fine-Tuning the DSP Controls

Fine-Tuning the Line Frequency Measurement

Line frequency measurement is based on zero-crossing

detection. For that purpose each voltage signal is

passed through a digital lowpass filter, controlled by

the ZC_LPF register. This register specifies the b0 coef-

ficient of a first-order LPF using following formula:

The MSB of this register must be zero.

For each phase A, B, and C, the MAXQ3183 counts the

number of scan frames (NS) between zero crossings

within a DSP cycle. Each individual phase A, B, or C

zero-crossing event contributes the raw NS count that

plugs as input to lowpass filter:

Yn = Yn - 1 + (AVG_NS/65,536) x (Xn - Yn - 1)

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.

Fundamental Mode Registers

The MAXQ3183 keeps another set of real and virtual

registers to track power and energy at the fundamental

line frequency. These “fundamental mode” registers

behave identically to the standard power and energy

registers, but are prefiltered to exclude harmonic

power.

The fundamental mode filter is specified in the B0FUND

and A1FUND registers. B0FUND is the feed-forward

coefficient and specifies the bandwidth of the funda-

mental mode filter; A1FUND is the feedback coefficient

and specifies the center frequency of the fundamental

mode filter.

In most cases, you can leave these filters at their

default values. If you wish to change the filter parame-

ters, first choose the desired bandwidth:

b0 =

π x bw x tFR

In this equation, bw is the desired bandwidth in hertz

and tFR is the frame period, typically 360μs. Set

B0FUND to b0 x 216. By default, B0FUND contains dec-

imal 145 (0x91) giving a bandwidth of about 2Hz.

To set the center frequency, calculate a1 according to

the following formula:

a1 = 2 - 2(1 - b0) x cos(2

π x fPK x tFR)

In this equation, b0 is the previously calculated feed-

forward coefficient, fPK is the desired center frequency

in hertz, and tFR is the ADC frame period. Set A1FUND

to a1 x 216. By default, A1FUND contains decimal 950

(0x3B6) giving a center frequency of approximately

50Hz.

The fundamental mode filter is, by default, quite sharp

with 3dB points only 1Hz off of the center frequency.

This means that if the frequency drifts even only slight-

ly, the fundamental mode power measurement is likely

to have significant inaccuracy.

The MAXQ3183 provides a mechanism to track the fre-

quency by updating the A1FUND register on each DSP

cycle. This mechanism is automatically enabled by

default.

You may wish to disable the automatic tracking facility

under some circumstances, particularly if you have

defined a broader bandwidth than default and are com-

fortable that the frequency will not drift beyond the

passband. To disable the filter, set the DFUNA bit in the

OPMODE2 register.

You may also elect to disable fundamental mode oper-

ation completely. To do this, set the DFUN bit in the

OPMODE2 register.

Harmonic Measurement

In addition to the ability to measure power and energy

at the fundamental frequency, the MAXQ3183 provides

a mechanism to isolate a particular harmonic on any

voltage or current channel and measure the amplitude

of that harmonic.

ZC LPF

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