GC5016 Datasheet, PDF (48/88 Pages) Texas Instruments – WIDEBAND QUAD DIGITAL DOWN CONVERTER/ UP CONVERTER

GC5016
SLWS142G − JANUARY 2003 − REVISED NOVEMBER 2005
14.9 The Sum Tree
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Figures 24 and 25 show the sum tree and transmit output format blocks when all four channels are added together,
including the sum IO path. Figure 24 shows the data paths for the standard 16 bit resolution mode (tout_res=0).
Figure 25 shows the wide resolution mode (tout_res=1).
The mixer outputs are rounded to 21–bits. This includes a 1 bit growth as the partial products of the complex multiply
are added together (effectively a gain relative to the msb of 0.5). The sum tree adds up to four up–conversion
channels together producing a 23–bit output (and a gain for any channel of 1/4). The 23–bit sum tree output is shifted
down by four bits and rounded to 19 bits before being added into the LSBs of the external 22–bit sum input. The gain
through the mixer, sumtree, and sumIO is equal to 2^−7. When sumIO is bypassed (no sum–in) or is in the 16–bit mode,
there is no shift down by the four bits, so the gain through the mixer, sumtree, and sumio are equal to 2^−3.
The final 22–bit sum is scaled up by 0 to 7 bits (sum_shift), checked for overflow, and then sent on to the transmit
output formatter. After scaling, the output data is rounded. Overflows in the sum tree are saturated to plus or minus
full scale. Hard limiting occurs after shifting and rounding. In normal applications, the data would be rounded to the
size of the digital to analog converter (DAC). The data is rounded into the uppermost bits and the unused lower bits
are cleared. For applications using sum inputs from other GC5016 chips, rounding should be to 22 bits for all chips
except the GC5016 just prior to the DAC.
The latency from the sum input port CO and DO to the output ports AO and BO is 14 clock cycles.
Sum In
21
I
Channel A
Q
21
21
I
Channel B
Q
21
21
I
Channel C
Q
21
Sum Tree
16
16
Upshift 7
23
23
23
23 Downshift 7
and
Upshift 0−7
23
23
and
Limit
sum_shift
Transmit
Output
Formatter
21
I
Channel D
Q
21
Figure 24. Sum Tree and Transmit Output Formatter − 16-Bit SumIO Mode
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