AD9278 datasheet(23/45 Pages) AD

23 / 45 page

AD9278

Data Sheet

Rev. A | Page 22 of 44

Table 8. Sensitivity and Dynamic Range of Trade-Offs1, 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 V4

GAIN+ = 1.6 V5

Input-Referred Noise6 at

GAIN+ = 1.6 V (nV/√Hz)

15.6

0.733

1.60

68.6

63.6

1.863

67.8

61.2

1.773

66.5

58.5

1.725

64.7

55.7

1.701

7.8

17.9

0.550

1.42

68.6

62.6

1.590

67.8

60.0

1.531

66.5

57.3

1.500

64.7

54.4

1.485

11.6

21.3

0.367

1.27

68.6

60.6

1.347

67.8

57.9

1.316

66.5

55.0

1.301

64.7

52.1

1.293

LNA: output full scale = 4.4 V p-p differential.

Filter: loss ~ 1 dB, NBW = 13.3 MHz, GAIN− = 0.8 V.

ADC: 40 MSPS, 70 dB SNR, 2 V p-p full-scale input.

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

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

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

0.4

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

AD9278 Datasheet(PDF) 23 Page - Analog Devices

AD9278 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Datasheet Download

Link URL