MAX78615+PPMSLASHC01T datasheet(16/46 Pages) MAXIM

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Line Frequency

This output is a measurement of the fundamental fre-

quency of the AC voltage source. It is derived from a

composite signal and therefore applies to all three phases

(it is a single reading per device) and is updated every 64

line cycles. Frequency data is reported as binary fixed-

point number, with a range of 0 to +256Hz less one LSB

(format S.16). See Table 14.

Current Channel Measurements

Instantaneous current measurements are updated every

sample, while peak currents and RMS currents are updat-

ed every accumulation interval (n samples). See Table 15.

Note that the INEUTRAL and IPHASE settings in the

CONFIG register affect how the instantaneous and aver-

aged values are computed as described in the Current

Input Configuration section.

Peak Current

This output is a capture of the largest magnitude instanta-

neous current load sample. See Figure 8.

RMS Current

The MAX78615+PPM reports true RMS measurements

for current inputs. The RMS current is obtained by per-

forming the sum of the squares of the instantaneous

voltage samples over the accumulation interval and then

performing a square root of the result after dividing by the

number of samples in the interval. See Figure 9.

An optional “RMS offset” for the current channels can be

adjusted to reduce errors due to noise or system offsets

(crosstalk) exhibited at low input amplitudes. Full-scale

values in the IxRMS_OFFS registers are squared and

subtracted from the accumulated/divided squares. If the

resulting RMS value is negative, zero is used.

Table 14. Frequency Measurement Register

Table 15. Current Channels Register

DESCRIPTION

LSB

TIME SCALE

FREQ

AC Voltage Frequency

Hz/216

64 voltage line cycles

DESCRIPTION

LSB

TIME SCALE

Instantaneous Current

FSI/223

1 sample

IA_PEAK

IB_PEAK

IC_PEAK

Peak Current

FSI/223

1 interval

IA_RMS

IB_RMS

IC_RMS

RMS Current

FSI/223

IT_RMS

Average of IA_RMS, IB_RMS, IC_RMS

FSI/223

Figure 8. Peak Current Value

Figure 9. True Current Input Value

INSTANTANEOUS

CURRENT (Ix)

ABS

MAX

MAXIMUM

Ix_PEAK

Ix2

Ix_RMS

÷ N

INSTANTANEOUS

CURRENT

N-1

∑

N=0

Ix2_SUM

—

IF |x|< 0

y = 0

IxRMS_OFF2

MAX78615+PPM

Isolated Energy Measurement Processor

for Polyphase Monitoring Systems

Maxim Integrated │ 16

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