Datasheet

AD9278 Data Sheet
Rev. A | Page 22 of 44
Table 8. Sensitivity and Dynamic Range of 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
at
GAIN+ = 1.6 V (nV/√Hz)
6 15.6 0.733 1.60 21 68.6 63.6 1.863
24 67.8 61.2 1.773
27 66.5 58.5 1.725
30 64.7 55.7 1.701
7.8 17.9 0.550 1.42 21 68.6 62.6 1.590
24 67.8 60.0 1.531
27
66.5
57.3
1.500
30 64.7 54.4 1.485
11.6 21.3 0.367 1.27 21 68.6 60.6 1.347
24 67.8 57.9 1.316
27 66.5 55.0 1.301
30 64.7 52.1 1.293
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 8 demonstrates the sensitivity and dynamic range of
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 + VGC) 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 prior to 0.1 dB compression on
the output of the LNA 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 40 to Figure 42.
1.2
0.8
1.0
0.6
0.4
0.2
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)
09424-048
PGA GAIN = 21dB
PGA GAIN = 24dB
PGA GAIN = 27dB
PGA GAIN = 30dB
Figure 40. LNA with 15.6 dB Gain Setting/VGA Full-Scale Limitations