Maxim Integrated 78M6631 - Manual (Page 13)

UG_6631_078 78M6631 Firmware Description Document
Rev 2 13
2.2.11 Sine and Cosine Reference
An internal Sine/Cosine oscillator generates waveforms used by the fundamental and harmonic
result calculations. The frequency of this oscillator is set by the FREQ variable. It is generally set to
the line frequency. The phase is unrelated and unimportant.
2.2.12 High-Pass Filters
An alternative to offset calibration is dynamic offset removal. This can be accomplished with either
lump-sum removal or a more gradual averaged reduction. The latter is similar to high-pass filtering,
but without phase distortion. All three methods are controlled by the HPF coefficient register. This is
a 9 bit value indicating how much of the measured offset (median) to remove each low-rate sample
period.
When HPFcoeff = 0, the last value in the offset registers is subtracted from the respective inputs.
This is the state that offset calibration uses. When HPFcoeff = 1.0 (0x100) the offset registers are
loaded with the measured offset (median) every low-rate sample. This is lump-sum offset removal.
When HPFcoeff is a fraction – say .75 (0xc0), 75% of the measured offset and 25% of the old
register value is used. Low values of HPFcoeff give very low HPF corner frequencies.