MAXQ3181-RAN+ Maxim Integrated Products, MAXQ3181-RAN+ Datasheet - Page 51

• Manufacturer Part Number: MAXQ3181-RAN+
• Description: IC AFE POLYPHASE LO-PWR 28-TSSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAXQ3181-RAN+

• Number Of Channels: 8
• Power (watts): 35mW
• Voltage - Supply, Analog: 3.3V
• Voltage - Supply, Digital: 3.3V
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Number Of Bits: -

crossing, affecting the accuracy of the energy mea-
sured during the preceding period. For this reason, a
second register, REJ_NS contains a value that specifies
how far a particular sample can deviate from the aver-
age and still be considered valid. If the period of the
newly acquired DSP cycle differs from the previously
accumulated average value by more than REJ_NS ADC
frames, NS is not updated with the new period (but the
energy is still accumulated).
With this discussion in mind, the signal path for the vari-
ous reported parameters can be reviewed.
RMS Volts and RMS Amps: First, the squared voltage
accumulation is divided by NS. This accomplishes the
“mean” part of the “root-mean-square” calculation.
Then, the square root of the result is taken, producing
the raw RMS calculation value.
On the voltage channel, the signal is ready for gain
compensation to be applied. But on the current channel,
there is an additional twist: depending on the amplitude
of the current, there may be a gain factor pre-applied
before the raw sample is available. To compensate for
inaccuracy in the gain factor for the amplifier and for
noise seen in the channel at high gain settings, it may
be necessary to provide linearity compensation.
There are three registers that manage the linearization
of the current signal: the X.OFFS_HI (X = A/B/C) regis-
ter contains a signed value that is added to the raw
RMS current signal before further processing; the
X.OFFS_LO register contains a signed value that is
added to the raw RMS current signal when the current
signal is below a low current threshold (1/32 of the full
scale) value; and the X.GAIN_LO register contains a
gain adjustment that is applied to the current signal
when the current signal is below the threshold value.
Figure 10. Computation of RMS Values
Low-Power, Active Energy, Polyphase AFE
______________________________________________________________________________________
NS
NS
V
I
2
2
LINEARIZATION
OFFS_LO
GAIN_LO
OFFS_HI
The practical effect of this is to turn what may be a
somewhat nonlinear response curve for the current sen-
sor to a much more linear response by two-piece
approximation.
The “high current” calibration term X.OFFS_HI is used
so long as the instantaneous current exceeds the low-
current threshold at some instant during a DSP cycle. As
long as this threshold is crossed during a DSP cycle, the
value in X.OFFS_HI controls the offset current.
When the input stays below the low-current threshold
for one DSP cycle, the X.OFFS_LO and X.GAIN_LO are
applied. The low-current calibration terms (X.GAIN_LO
and X.OFFS_LO) remain in effect until the peak of input
current waveform exceeds 1/32 of full-scale current at
any time during a DSP cycle.
As a final step, both voltage and current are passed
through an averaging filter that provides smoothing for
the signals. The amount of filtering is given in AVG_C.
Energy: Current sensors and other external circuitry
components could introduce a phase distortion to the
current signal, and this phase distortion may not be
constant at all current values. Consequently, for the
most precise measurements, the phase between the
voltage and current signals must be compensated. In
the MAXQ3181, the energy signals are compensated
for phase offset by performing a complex multiplication
of the signal with the contents of the appropriate phase
offset register.
Determining which phase offset register is appropriate is
a matter of comparing the incoming RMS current for the
phase with the contents of the I1THR and I2THR regis-
ters. It is the responsibility of the administrative software
to ensure that I1THR is greater than or equal to I2THR. If
the raw RMS current is greater than or equal to the con-
AVERAGE
AVERAGE
AVG_C
AVG_C
I
RAW_I
RAW_V
V
RMS
RMS
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