Datasheet

AD9279
Rev. 0 | Page 21 of 44
In its default condition, the LNA has a gain of 21.3 dB (12×), and
the VGA postamp gain is 24 dB. If the voltage on the GAIN+
pin is 0 V and the voltage on the GAIN− pin is 0.8 V (44.8 dB
attenuation), the total gain of the channel is 0.5 dB if the LNA
input is unmatched. The channel gain is −5.5 dB if the LNA is
matched to 50 Ω (R
FB
= 350 Ω). However, if the voltage on the
GAIN+ pin is 1.6 V and the voltage on the GAIN− pin is 0.8 V
(0 dB attenuation), the VGA
ATT
is 0 dB. This results in a total
gain of 45.3 dB through the TGC path if the LNA input is
unmatched or in a total gain of 39.3 dB if the LNA input is
matched.
LNA FULL SCALE
(0.450V p-p SINGLE-ENDED)
LNA INPUT-REFERRED
NOISE FLOOR
(3.1µV rms) AT AAF BW = 15MHz
LNA + VGA NOISE = 0.8nV/ Hz
A
DC FULL SCALE (2V p-p)
~6dB MARGIN
>10dB MARGIN
ADC NOISE FLOOR
(224µV rms)
MINIMUM GAIN
MAXIMUM GAIN
LNA
ADC
70dB
VGA GAIN RANGE > 38dB
MAX CHANNEL GAIN > 47dB
94dB
09423-047
Figure 42. Gain Requirements of TGC Operation for a 12-Bit, 40 MSPS ADC