Cirrus Logic AN336 - Manual (Page 2)

2 AN336REV1
AN336
In power meter applications, there are strict requirements for the limit of reactive power error due to line fre-
quency variation. The AFC function can keep the Epsilon register automatically updated with line frequency
changes to ensure accurate reactive power calculations.
If the measurement delay is not important to the application, the AFC function can be used to measure the
line frequency as well.
Line Frequency = Epsilon × OWR
4. PRECAUTIONS WHEN USING THE AFC IN THE CS5463 REVISION D
In revision D of the CS5463, the HPF settling time has been increased to 3000 samples or 3000/OWR sec-
onds (see the CS5463 Technical Bulletin). However, this settling time was not taken into account by the
automatic Epsilon calculation. This may make the 1
st
Epsilon and Qavg results inaccurate if the AFC is en-
abled before HPF has settled. See Figure 2.
Figure 2. HPF Settling Time – AFC Enabled Before Start of Conversion
To solve this problem, the AFC should be enabled at least 3000/OWR seconds after the continuous conversion has
started. See Figure 3.
Figure 3. HPF Settling Time – AFC Enabled After HPF Has Settled
5. CONCLUSION
The Epsilon register plays an important role in the CS5463 reactive power and line frequency calculations.
To avoid inaccurate measurements, the Epsilon register should be preset based on the line frequency, and
the AFC should be enabled with some delay after the start of continuous conversions to allow the CS5463
to automatically track any line frequency variation.
Start of Continuous
Conversion
Initial Epsilon
(By default or preset)
2 nd Update Period 3 rd Update Period 1 st Update Period
Inaccurate Epsilon
& Qavg
Accurate Epsilon
& Qavg
HPF settling time
Enable AFC
Start of Continuous
Conversion
Initial Epsilon
(By default or preset)
2
nd
Update Period 3
rd
Update Period 1
st
Update Period
Accurate Epsilon
& Qavg
Accurate Epsilon
& Qavg
HPF settling time
Enable AFC