AD9277 datasheet(32/49 Pages) AD

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AD9277

Rev. 0 | Page 31 of 48

0.6

0.5

0.4

0.3

0.2

0.1

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 57. LNA with 17.9 dB Gain Setting/VGA Full-Scale Limitations

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

GAIN+ (V)

PGA GAIN = 21dB

PGA GAIN = 30dB

PGA GAIN = 27dB

PGA GAIN = 24dB

Figure 58. LNA with 21.3 dB Gain Setting/VGA Full-Scale Limitations

Variable Gain Amplifier (VGA)

The differential X-AMP VGA provides precise input attenu-

ation and interpolation. It has a low input-referred noise of

3.8 nV/√Hz and excellent gain linearity. A simplified block

diagram is shown in Figure 59.

VIP

GAIN±

3.5dB

VIN

POSTAMP

–

GAIN INTERPOLATOR

Figure 59. Simplified VGA Schematic

The input of the VGA is a 14-stage differential resistor ladder with

3.5 dB per tap. The resulting total gain range is 42 dB, which

allows for range loss at the endpoints. The effective input resistance

per side is 180 Ω nominally for a total differential resistance of

360 Ω. The ladder is driven by a fully differential input signal from

the LNA. LNA outputs are dc-coupled to avoid external decoupling

capacitors. The common-mode voltage of the attenuator and the

VGA is controlled by an amplifier that uses the same midsupply

voltage derived in the LNA, permitting dc coupling of the LNA

to the VGA without introducing large offsets due to common-

mode differences. However, any offset from the LNA becomes

amplified as the gain increases, producing an exponentially

increasing VGA output offset.

The input stages of the X-AMP are distributed along the ladder,

and a biasing interpolator, controlled by the gain interface, deter-

mines the input tap point. With overlapping bias currents, signals

from successive taps merge to provide a smooth attenuation range

from −42 dB to 0 dB. This circuit technique results in linear-in-dB

gain law conformance and low distortion levels—only deviating

±0.5 dB or less from the ideal. The gain slope is monotonic with

respect to the control voltage and is stable with variations in

process, temperature, and supply.

The X-AMP inputs are part of a programmable gain feedback

amplifier that completes the VGA. Its bandwidth is approximately

100 MHz. The input stage is designed to reduce feedthrough to

the output and to ensure excellent frequency response uniformity

across the gain setting.

Gain Control

The gain control interface, GAIN±, is a differential input. VGAIN

varies the gain of all VGAs through the interpolator by selecting

the appropriate input stages connected to the input attenuator.

For GAIN− at 0.8 V, the nominal GAIN+ range for 28.5 dB/V is

0 V to 1.6 V, with the best gain linearity from about 0.16 V to

1.44 V, where the error is typically less than ±0.5 dB. For GAIN+

voltages greater than 1.44 V and less than 0.16 V, the error

increases. The value of GAIN+ can exceed the supply voltage

by 1 V without gain foldover.

Gain control response time is less than 750 ns to settle within 10%

of the final value for a change from minimum to maximum gain.

The GAIN+ and GAIN− pins can be interfaced in one of two

ways. Using a single-ended method, a Kelvin type of connec-

tion to ground can be used, as shown in Figure 60. For driving

multiple devices, it is preferable to use a differential method, as

shown in Figure 61. In either method, the GAIN+ and GAIN−

pins should be dc-coupled and driven to accommodate a 1.6 V

full-scale input.

GAIN+

GAIN–

100Ω

0V TO 1.6V DC

50Ω

0.01µF

KELVIN

CONNECTION

Figure 60. Single-Ended GAIN+, GAIN− Pin Configuration

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