Datasheet

LM97593
www.ti.com SNWS019B –JULY 2007–REVISED APRIL 2013
Figure 64. Complex NCO Output Phase Dither Enabled
Worst Case Amplitude Spur, NCO at F
S
/8
The CIC filter has a gain equal to N
4
(filter decimation^4) which must be compensated for in the “SHIFT UP”
circuit shown in Figure 65 as well as Figure 60. This circuit has a gain equal to 2
(SCALE-44)
, where SCALE ranges
from 0 to 40. This circuit divides the input signal by 2
44
providing
Figure 65. Four-stage decimate by N CIC filter
maximum headroom through the CIC filter. For optimal noise performance the SCALE value is set to increase
this level until the CIC filter is just below the point of distortion. A value is normally calculated and loaded for
SCALE such that GAIN
SHIFTUP
*GAIN
CIC
≤ 1. The actual gain of the CIC filter will only be unity for power-of-two
decimation values. In other cases the gain will be somewhat less than unity.
Channel Gain
The gain of each channel can be boosted up to 42 dB by shifting the output of the CIC filter left by 0 to 7 bits
prior to rounding it to 21 bits. For channel A, the gain of this stage is: GAIN = 2
GAIN_A
, where GAIN_A ranges
from 0 to 7. Overflow due to the GAIN circuit is saturated (clipped) at plus or minus full scale. Each channel can
be given its own GAIN setting.
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