MAXQ3181-RAN+ datasheet(62/84 Pages) MAXIM

62 / 84 page

Low-Power, Active Energy, Polyphase AFE

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the load resistor is 600

Ω and that the impedance of the

transformer secondary is 200

Ω. The resistor required in

the primary is

(600 + 200) x 500 = 400k

Often, this resistor is constructed from multiple

instances of a smaller value resistor; in this case, one

might use eight 50k

Ω resistors. Doing so minimizes the

voltage requirements for the resistor chain and reduces

the possibility that a single point of failure will cause a

catastrophic failure.

Current Sensors

Current Shunt

A current shunt is a low-value (approximately 100μ

Ω to

a 100m

Ω) resistor that converts a large-value current

into a small voltage. Shunts make good current sensors

because the output is an extremely linear representa-

tion of the measured current, current shunts can have

very low temperature coefficients, and they are inex-

pensive.

The power dissipated by a current shunt is inversely

proportional to its resistance and proportional to the

square of the output voltage. Consequently, there is

great incentive to reduce the resistance (and hence,

the output voltage) of a shunt. Often, full-scale current

in a shunt produces only a few millivolts of output, mak-

ing a front-end amplifier essential. The MAXQ3181

includes a gain-of-32 amplifier in the current channels

that is automatically cycled in and out, depending on

the input voltage of the current channels.

Current shunts operate at line voltage, thus, the AFE

must be isolated from the line. That means that in a

wye-connected meter, the current sensing must be per-

formed in the neutral return circuit (so that all voltages

into the current-sense amplifiers are referenced to neu-

tral). It also means that the use of a shunt is precluded

for delta-connected meters; the MAXQ3181 cannot tol-

erate the line-voltage differential between channels.

Current Transformer

In a current transformer, the primary is usually one turn

of thick wire or buss bar and the secondary is often

1000 turns or more of magnet wire. A ferrite core mag-

netically couples the two. Thus, a large current in the

primary turn creates a small current but large voltage in

the secondary winding.

For example, assume a current transformer with a 1000

turn secondary. A 10A current in the primary winding

induces a 10mA current in the secondary. This current

is made to flow through a so-called “burden” resistor,

usually 10

Ω to 20Ω. Assuming a 20Ω burden, our 10A

current thus produces a 200mV signal in the secondary.

Advanced Operation

Modifying the ADC Operation

There are several other registers that directly affect the

AFE function. These registers directly affect the hard-

ware functionality, and should be modified only when it

is explicitly required. For example, if the MAXQ3181 is

operated at some frequency other than the nominal

8MHz system clock, modification of these registers by

supervisory code becomes necessary to maintain a

320μs frame time.

• R_ACFG: This register contains bits that disable the

ADC entirely, disable the voltage reference buffer

amplifier, and disable the ADC interrupt. Modifying

this register will likely disable or impair operation of

the MAXQ3181 internal firmware.

• R_ADCRATE: Modify this register to change the rate

at which the MAXQ3181 acquires samples. By

default, R_ADCRATE contains 319 decimal, which

means that the ADC acquires a sample every 320

system clocks. With an 8MHz clock, this translates to

40μs. If the system clock is slower, it may be advan-

tageous to reduce this value to keep a 40μs per sam-

ple time constant.

• 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 MAXQ3181 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)

ZC LPF

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