ADS6122 Datasheet, PDF (46/67 Pages) Texas Instruments – 12-BITS, 125/105/80/65 MSPS ADC WITH DDR LVDS/CMOS OUTPUTS

ADS6125, ADS6124
ADS6123, ADS6122
SLAS560A – OCTOBER 2007 – REVISED MARCH 2008
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COARSE GAIN and PROGRAMMABLE FINE GAIN
ADS612X includes gain settings that can be used to get improved SFDR performance (compared to 0 dB gain
mode). The gain settings are 3.5 dB coarse gain and programmable fine gain from 0 dB to 6 dB. For each gain
setting, the analog input full-scale range scales proportionally, as shown in Table 15.
The coarse gain is a fixed setting of 3.5 dB and is designed to improve SFDR with little degradation in SNR (as
seen in Figure 13 and Figure 14). The fine gain is programmable in 1 dB steps from 0 to 6 dB. With fine gain
also, SFDR improvement is achieved, but at the expense of SNR (there is about 1 dB SNR degradation for every
1 dB of fine gain).
So, the fine gain can be used to trade-off between SFDR and SNR. The coarse gain makes it possible to get
best SFDR but without losing SNR significantly. At high input frequencies, the gains are especially useful as the
SFDR improvement is significant with marginal degradation in SINAD. The gains can be programmed using the
register bits <COARSE GAIN> (see Table 5) and <FINE GAIN> (see Table 10). Note that the default gain after
reset is 0 dB.
GAIN, dB
0
3.5
1
2
3
4
5
6
Table 15. Full-Scale Range Across Gains
TYPE
Default after reset
Coarse setting (fixed)
Fine gain (programmable)
FULL-SCALE RANGE, V_PP
2.00
1.34
1.78
1.59
1.42
1.26
1.12
1.00
46
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