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AD9272

Rev. C | Page 28 of 44

Table 9. Sensitivity and Dynamic Range of Trade-Offs1, 2, 3

LNA

VGA

Channel

Gain

Full-Scale Input

(V p-p)

Input-Referred

Noise Voltage

(nV/√Hz)

Postamp Gain (dB)

Typical Output Dynamic Range

Input-Referred Noise4 @

GAIN+ = 1.6 V (nV/√Hz)

(V/V)

(dB)

GAIN+ = 0 V5

GAIN+ = 1.6 V6

15.6

0.733

0.98

67.5

65.1

1.395

66.4

63.0

1.286

64.6

60.6

1.227

62.5

57.9

1.197

17.9

0.550

0.86

67.5

64.5

1.149

66.4

62.3

1.071

64.5

59.8

1.030

62.5

57.1

1.009

21.3

0.367

0.75

67.5

63.3

0.910

66.4

60.9

0.865

64.6

58.2

0.842

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 Channel noise at maximum VGA gain.

5 Output dynamic range at minimum VGA gain (VGA dominated).

6 Output dynamic range at maximum VGA gain (LNA dominated).

Table 9 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 perfor-

mance (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± 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 47 to

Figure 49.

0.1

0.2

0.3

0.4

0.5

0.6

0.8

0.7

0.9

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

GAIN+ (V)

PGA GAIN = 21dB

PGA GAIN = 24dB

PGA GAIN = 27dB

PGA GAIN = 30dB

Figure 47. LNA with 15.6 dB Gain Setting/VGA Full-Scale Limitations

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