MAXQ3181 Maxim, MAXQ3181 Datasheet - Page 62

• Manufacturer Part Number: MAXQ3181
• Description: The MAXQ3181 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, active power and energy, and many other metering parameters of a polyphase load
• Manufacturer: Maxim

Low-Power, Active Energy, Polyphase AFE
the load resistor is 600Ω and that the impedance of the
transformer secondary is 200Ω. The resistor required in
the primary is
Often, this resistor is constructed from multiple
instances of a smaller value resistor; in this case, one
might use eight 50kΩ 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.
A current shunt is a low-value (approximately 100μΩ to
a 100mΩ) 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.
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 10Ω to 20Ω. Assuming a 20Ω burden, our 10A
current thus produces a 200mV signal in the secondary.
62
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(600 + 200) x 500 = 400kΩ
Current Transformer
Current Sensors
Current Shunt
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
• R_ADCRATE: Modify this register to change the rate
• R_ADCACQ: Modify this register to change the
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 b
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:
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.
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.
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.
Y
Fine-Tuning the Line Frequency Measurement
n
= Y
n - 1
+ (AVG_NS/65,536) x (X
Fine-Tuning the DSP Controls
Modifying the ADC Operation
b
Advanced Operation
0
=
ZC LPF
2
_
16
n
- Y
n - 1
0
)
coef-