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AD9271

Preliminary Technical Data

Rev. PrA | Page 26 of 58

gain error and uniform bandwidth, and differential signal paths

minimize distortion.

At low gain the VGA should limit the system noise performance

(SNR), whereas at high gains the noise is defined by the source

and LNA. The maximum voltage swing is bounded by the full-

scale peak-to-peak ADC input voltage (2 V p-p).

Variable Gain Amplifier

The differential X-AMP VGA provides precise input

attenuation and interpolation. It has a low input-referred noise

of 4 nV/√Hz and excellent gain linearity. A simplified block

diagram is shown in Figure 42.

VIP

GAIN

3dB

VIN

POSTAMP

–

GAIN INTERPOLATOR

Figure 42. Simplified VGA Schematic

The input of the VGA is a 12-stage differential resistor ladder with

3.01 dB per tap. The resulting total gain range is 30 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 will be

amplified as the gain is increased, 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,

determines the input tap point. With overlapping bias currents,

signals from successive taps merge to provide a smooth

attenuation range from 0 dB to −30 dB. This circuit technique

results in linear-in-dB gain law conformance and low distortion

levels—only deviating ±0.2 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 25 dB gain feedback amplifier

that completes the VGA. Its bandwidth is about 80 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.

The nominal VGAIN range for 30 dB/V is 0 V to 1 V, with the best

gain-linearity from about 0.1 V to 0.9 V, where the error is

typically less than ±0.2 dB. For VGAIN voltages greater than 0.9 V

and less than 0.1 V, the error increases. The value of the VGAIN

voltage can be increased to that of the supply voltage 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.

There are two ways in which the GAIN pins can be interfaced.

Using a single-ended method, a Kelvin type of connection to

ground should be used as shown in Figure 43. For driving multiple

devices, it is preferred to use a differential method as shown in

Figure 44. In either method, the GAIN pins should be dc-coupled

and driven to accommodate a 1 V full-scale input.

Figure 43. Single-Ended Gain Pin Configuration

GAIN–

50Ω

GAIN+

AD9271

AVDD

26kΩ

10kΩ

0.01µF

±0.25DC AT

0.5V CM

±0.25DC AT

0.5V CM

100Ω

499Ω

±0.5V DC

0.01µF

100Ω

499Ω

523Ω

499Ω

0.5V CM

AD8318

Figure 44. Differential Gain Pin Configuration

VGA Noise

In a typical application, a VGA compresses a wide dynamic

range input signal to within the input span of an ADC. The

input-referred noise of the LNA limits the minimum resolvable

input signal, whereas the output-referred noise, which depends

primarily on the VGA, limits the maximum instantaneous

dynamic range that can be processed at any one particular gain

control voltage. This limit is set in accordance with the

quantization noise floor of the ADC.

Output- and input-referred noise as a function of VGAIN are

shown in Figure TBD and Figure TBD for the short-circuited

input conditions. The input noise voltage is simply equal to the

output noise divided by the measured gain at each point in the

control range.

The output-referred noise is a flat 65 nV/√Hz over most of the

gain range, because it is dominated by the fixed output-referred

noise of the VGA. At the high end of the gain control range, the

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