Appendix A: Applying a Lowpass
Filter (LPF) for Application-
Optimized ADC Performance
The MAXQ3120 gives a user application program direct
access to the ADC data stream right after its sinc3 fil-
ters. This unique feature permits the MAXQ3120 to be
optimized for its target applications. With the device’s
8MIPS processing power and the 1-cycle MAC, a linear
FIR filter can be easily computed in the user application
program. This section provides a simple LP FIR filter to
improve the SNR and THD performance of the
MAXQ3120 for the power-metering application.
Filter Specifications and Coefficients
Input signal frequency (f
IN
) = 60Hz
Sampling frequency (f
S
) = 20833Hz
Window = hamming
Cutoff frequency (attenuates after 7th harmonic) = 0.06π
= 625Hz
Cutoff frequency (for 21st harmonic) = 0.18π = 1875Hz
Transition width = 0.35π = 3646Hz
Filter length = 23
MAXQ3120
High-Precision ADC
Mixed-Signal Microcontroller
____________________________________________________________________ 29
FILTER_COEFFICIENT_0 -19 FILTER_COEFFICIENT_12 21893
FILTER_COEFFICIENT_1 -288 FILTER_COEFFICIENT_13 17547
FILTER_COEFFICIENT_2 -786 FILTER_COEFFICIENT_14 11739
FILTER_COEFFICIENT_3 -1402 FILTER_COEFFICIENT_15 6018
FILTER_COEFFICIENT_4 -1670 FILTER_COEFFICIENT_16 1627
FILTER_COEFFICIENT_5 -876 FILTER_COEFFICIENT_17 -876
FILTER_COEFFICIENT_6 1627 FILTER_COEFFICIENT_18 -1670
FILTER_COEFFICIENT_7 6018 FILTER_COEFFICIENT_19 -1402
FILTER_COEFFICIENT_8 11739 FILTER_COEFFICIENT_20 -786
FILTER_COEFFICIENT_9 17547 FILTER_COEFFICIENT_21 -288
FILTER_COEFFICIENT_10 21893 FILTER_COEFFICIENT_22 -19
FILTER_COEFFICIENT_11 23506 — —
FILTER_COEFFICIENT_0 849 FILTER_COEFFICIENT_12 23295
FILTER_COEFFICIENT_1 1420 FILTER_COEFFICIENT_13 21720
FILTER_COEFFICIENT_2 2553 FILTER_COEFFICIENT_14 19291
FILTER_COEFFICIENT_3 4334 FILTER_COEFFICIENT_15 16269
FILTER_COEFFICIENT_4 6754 FILTER_COEFFICIENT_16 12966
FILTER_COEFFICIENT_5 9700 FILTER_COEFFICIENT_17 9700
FILTER_COEFFICIENT_6 12966 FILTER_COEFFICIENT_18 6754
FILTER_COEFFICIENT_7 16269 FILTER_COEFFICIENT_19 4334
FILTER_COEFFICIENT_8 19291 FILTER_COEFFICIENT_20 2553
FILTER_COEFFICIENT_9 21720 FILTER_COEFFICIENT_21 1420
FILTER_COEFFICIENT_10 23295 FILTER_COEFFICIENT_22 849
FILTER_COEFFICIENT_11 23840 — —
1) LPF coefficients (up to 21st harmonic). Note that these coefficients have been converted to 16-bit fixed-point
numbers. A shift of 17 places is required after the multiply-accumulate operation.
2) LPF coefficients (up to 7th harmonic). Note that these coefficients have been converted to 16-bit fixed-point
numbers. A shift of 18 places is required after the multiply-accumulate operation.
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