Datasheet

AD9276
Rev. 0 | Page 30 of 48
Table 10. Sensitivity and Dynamic Range Trade-Offs
1, 2 , 3
LNA VGA Channel
Gain
Full-Scale
Input (V p-p)
Input Noise
(nV/√Hz) Postamp Gain (dB)
Typical Output Dynamic Range (dB)
(V/V) (dB) GAIN+ = 0 V
4
GAIN+ = 1.6 V
5
Input-Referred Noise
6
@
GAIN+ = 1.6 V (nV/√Hz)
6 15.6 0.733 0.98 21 67.5 65.1 1.395
24 66.4 63.0 1.286
27 64.6 60.6 1.227
30 62.5 57.9 1.197
8 17.9 0.550 0.86 21 67.5 64.5 1.149
24 66.4 62.3 1.071
27 64.6 59.8 1.030
30 62.5 57.1 1.009
12 21.3 0.367 0.75 21 67.5 63.3 0.910
24 66.4 60.9 0.865
27 64.6 58.2 0.842
30 62.5 55.4 0.830
1
LNA: output full scale = 4.4 V p-p differential.
2
Filter: loss ≈ 1 dB, NBW = 13.3 MHz, GAIN− = 0.8 V.
3
ADC: 40 MSPS, 70 dB SNR, 2 V p-p full-scale input.
4
Output dynamic range at minimum VGA gain (VGA dominated).
5
Output dynamic range at maximum VGA gain (LNA dominated).
6
Channel noise at maximum VGA gain.
Table 10 demonstrates the sensitivity and dynamic range
trade-offs that can be achieved relative to various LNA and
VGA gain settings.
For example, when the VGA is set for the minimum gain voltage,
the TGC path is dominated by VGA noise and achieves the
maximum output SNR. However, as the postamp gain options
are increased, the input-referred noise is reduced and the SNR
is degraded.
If the VGA is set for the maximum gain voltage, the TGC path
is dominated by LNA noise and achieves the lowest input-
referred noise, but with degraded output SNR. The higher the
TGC (LNA + VGA) gain, the lower the output SNR. As the
postamp gain is increased, the input-referred noise is reduced.
At low gains, the VGA should limit the system noise performance
(SNR); at high gains, the noise is defined by the source and the
LNA. The maximum voltage swing is bound by the full-scale
peak-to-peak ADC input voltage (2 V p-p).
Both the LNA and VGA have full-scale limitations within each
section of the TGC path. These limitations are dependent on the
gain setting of each function block and on the voltage applied to
the GAIN+ and GAIN− pins. The LNA has three limitations,
or full-scale settings, that can be applied through the SPI.
Similarly, the VGA has four postamp gain settings that can be
applied through the SPI. The voltage applied to the GAIN± pins
determines which amplifier (the LNA or VGA) saturates first.
The maximum signal input level that can be applied as a
function of voltage on the GAIN± pins for the selectable gain
options of the SPI is shown in Figure 56 to Figure 58.
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
GAIN+ (V)
INPUT FULL SCALE (V p-p)
08180-050
PGA GAIN = 21dB
PGA GAIN = 24dB
PGA GAIN = 27dB
PGA GAIN = 30dB
Figure 56. LNA with 15.6 dB Gain Setting/VGA Full-Scale Limitations