MAXQ3180 Maxim, MAXQ3180 Datasheet - Page 69

• Manufacturer Part Number: MAXQ3180
• Description: The MAXQ3180 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, power, energy, and many other metering and power-quality parameters of a polyphase load
• Manufacturer: Maxim

The second consideration is the total power dissipation
and voltage hold-off requirements of the resistor. It is
tempting to design a 400:1 divider with a 400kΩ resis-
tor in series with a 1kΩ resistor, but that would force the
400kΩ resistor to dissipate about 140mW. This is not an
excessive amount of power, but if the design is to use
small SMT parts, it can handle greater than a 1/10W
SMT resistor. It is better to use a series of several small-
er components to improve system reliability.
If isolation is required between the meter electronics
and the line, a voltage transformer is required. A volt-
age transformer is designed to faithfully transfer an AC
voltage applied on the primary side to a sensor on the
secondary side. On the primary side, a voltage-divider
is used to reduce the voltage to a workable level. On
the secondary side, a load resistor is selected so that
the current in the transformer windings is safely within
the transformer’s linear operating region.
Because the impedance seen in the primary side of the
transformer is equal to the impedance of the load resis-
tor in the secondary circuit plus impedance of the
transformer secondary winding at the operating fre-
quency, it is easy to calculate the value of the required
voltage-divider resistors in the primary side. For exam-
ple, assume we want a 500:1 divider ratio and assume
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
(600 + 200) x 500 = 400kΩ
Low-Power, Multifunction, Polyphase AFE
______________________________________________________________________________________
Voltage Transformer
Current Sensors
Current Shunt
in a shunt produces only a few millivolts of output, mak-
ing a front-end amplifier essential. The MAXQ3180
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 MAXQ3180 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.
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 MAXQ3180 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
ADC entirely, disable the voltage reference buffer
amplifier, and disable the ADC interrupt. Modifying
this register will likely disable or impair operation of
the MAXQ3180 internal firmware.
at which the MAXQ3180 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.
Modifying the ADC Operation
Advanced Operation
Current Transformer
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