LM97593VH/HALF National Semiconductor, LM97593VH/HALF Datasheet - Page 30

• Manufacturer Part Number: LM97593VH/HALF
• Manufacturer: National Semiconductor

Specifications of LM97593VH/HALF

• Lead Free Status / Rohs Status: Compliant

www.national.com
GAIN
only be unity for power-of-two decimation values. In other
cases the gain will be somewhat less than unity.
4.3 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
due to the GAIN circuit is saturated (clipped) at plus or minus
full scale. Each channel can be given its own GAIN setting.
4.4 First Programmable FIR Filter (F1)
The CIC/GAIN outputs are followed by two stages of filtering.
The first stage is a 21 tap decimate-by-2 symmetric FIR filter
with programmable coefficients. Typically, this filter compen-
sates for a slight droop induced by the CIC filter while remov-
ing undesired alias images above Nyquist. In addition, it often
provides stopband assistance to F2 when deep stop bands
are required. The filter coefficients are 16-bit 2’s complement
numbers. Unity gain will be achieved through the filter if the
sum of the 21 coefficients is equal to 2
2
2
= 0, ..., 20 where h
symmetric, so only the first 11 are loaded into the chip.
Two example sets of coefficients are provided here. The first
set of coefficients, referred to as the standard set (STD), com-
pensates for the droop of the CIC filter providing a passband
which is flat (0.01 dB ripple) over 95% of the final output
bandwidth with 70dB of out-of-band rejection (see Figure
25). The filter has a gain of 0.999 and is symmetric with the
following 11 unique taps (1|21, 2|20, ..., 10|12, 11):
The second set of coefficients (GSM set) are intended for ap-
plications that need deeper stop bands or need oversampled
outputs. These requirements are common in cellular systems
where out of band rejection requirements can exceed 100dB
(see Figure 26). They are useful for wideband radio architec-
tures where the channelization is done after the ADC. These
filter coefficients introduce a gain of 0.984 and are:
16
16
, then F1 will introduce a gain equal to (sum of coefficients)/
. The 21 coefficients are identified as coefficients h
SHIFTUP
29, -85, -308, -56, 1068, 1405, -2056, -6009,
1303, 21121, 32703
-49, -340, -1008, -1617, -1269, 425, 3027, 6030,
9115, 11620, 12606
FIGURE 25. F1 STD frequency response
GAIN_A
*GAIN
, where GAIN_A ranges from 0 to 7. Overflow
1
CIC
(10) is the center tap. The coefficients are
≤
1. The actual gain of the CIC filter will
16
. If the sum is not
30008723
1
(n), n
30
4.5 Second Programmable FIR Filter (F2)
The second stage decimate by two or four filter also uses ex-
ternally downloaded filter coefficients. F2 determines the final
channel filter response. The filter coefficients are 16-bit 2’s
complement numbers. Unity gain will be achieved through the
filter if the sum of the 63 coefficients is equal to 2
is not 2
coefficients)/2
The 63 coefficients are identified as h
h
only the first 32 are loaded into the chip. An example filter
(STD F2 coefficients, see Figure 27) with 80dB out-of-band
rejection, gain of 1.00, and 0.03 dB peak to peak passband
ripple is created by this set of 32 unique coefficients:
A second set of F2 coefficients (GSM set, see Figure 28) suit-
able for meeting the stringent wideband GSM requirements
with a gain of 0.999 are:
The filter coefficients of both filters can be used to tailor the
spectral response to the user’s needs. For example, the first
can be loaded with the standard set to provide a flat
2
(31) is the center tap. The coefficients are symmetric, so
-14, -20, 19, 73, 43, -70, -82, 84, 171, -49, -269,
-34, 374, 192, -449,
-430, 460,751, -357, -1144, 81, 1581, 443, -2026,
-1337, 2437, 2886,
-2770, -6127, 2987, 20544, 29647
-536, -986, 42, 962, 869, 225, 141, 93, -280,
-708, -774, -579, -384,
-79, 536, 1056, 1152, 1067, 789, 32, -935, -1668,
-2104, -2137, -1444,
71, 2130, 4450, 6884, 9053, 10413, 10832
16
FIGURE 26. F1 GSM frequency response
, then the F2 will introduce a gain equal to (sum of
16
.
2
(n), n = 0, ..., 62 where
16
30008724
. If the sum