AD8331 datasheet(20/32 Pages) AD

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AD8331/AD8332

Rev. C | Page 20 of 32

X-AMP VGA

The input of the VGA is a differential R-2R ladder attenuator

network, with 6 dB steps per stage and a net input impedance of

200 Ω differential. The ladder is driven by a fully differential

input signal from the LNA and is not intended for single-ended

operation. LNA outputs are ac-coupled to reduce offset and

isolate their common-mode voltage. The VGA inputs are biased

through the ladder’s center tap connection to VCM, which is

typically set to 2.5 V and is bypassed externally to provide a

clean ac ground.

The signal level at successive stages in the input attenuator falls

from 0 dB to –48 dB, in 6 dB steps. 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 –48 dB. This

circuit technique results in excellent, linear-in-dB gain law

conformance and low distortion levels and deviates ±0.2 dB or less

from 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 gain-of-12 feedback amplifier,

which completes the VGA. Its bandwidth is 150 MHz. The input

stage is designed to reduce feedthrough to the output and

ensure excellent frequency response uniformity across gain

setting (see Figure 8 and Figure 9).

Gain Control

Position along the VGA attenuator is controlled by a single-

ended analog control voltage, VGAIN, with an input range of 40 mV

to 1.0 V. The gain control scaling is trimmed to a slope of 50 dB/V

(20 mV/dB). Values of VGAIN beyond the control range saturate to

minimum or maximum gain values. Both channels of the

AD8332 are controlled from a single gain interface to preserve

matching. Gain can be calculated using Equations 1 and 2.

Gain accuracy is very good since both the scaling factor and

absolute gain are factory trimmed. The overall accuracy relative

to the theoretical gain expression is ±1 dB for variations in

temperature, process, supply voltage, interpolator gain ripple, trim

errors, and tester limits. The gain error relative to a best-fit line for

a given set of conditions is typically ±0.2 dB. Gain matching

between channels is better than 0.1 dB (see Figure 7, which shows

gain errors in the center of the control range). When VGAIN < 0.1

or > 0.95, gain errors are slightly greater.

The gain slope may be inverted, as shown in Figure 58 (avail-

able in most versions). The gain drops with a slope of

–50 dB/V across the gain control range from maximum to

minimum gain. This slope is useful in applications, such as

automatic gain control, where the control voltage is

proportional to the measured output signal amplitude. The

inverse gain mode is selected by setting the MODE pin HI.

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

10% of the final value for a change from minimum to max-

imum gain.

VGA Noise

In a typical application, a VGA compresses a wide dynamic

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

the input-referred noise of the LNA limits the minimum

resolvable input signal, 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

plotted in Figure 21 and Figure 23 for the short-circuited input

condition. 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 flat over most of the gain range,

since it is dominated by the fixed output-referred noise of the

VGA. Values are 48 nV/√Hz in LO gain mode and 178 nV/√Hz

in HI gain mode. At the high end of the gain control range, the

noise of the LNA and source prevail. The input-referred noise

reaches its minimum value near the maximum gain control

voltage, where the input-referred contribution of the VGA

becomes very small.

At lower gains, the input-referred noise, and thus noise figure,

increases as the gain decreases. The instantaneous dynamic

range of the system is not lost, however, since the input capacity

increases with it. The contribution of the ADC noise floor has

the same dependence as well. The important relationship is

the magnitude of the VGA output noise floor relative to that

of the ADC.

With its low output-referred noise levels, these devices ideally

drive low-voltage ADCs. The converter noise floor drops 12 dB

for every 2 bits of resolution and drops at lower input full-scale

voltages and higher sampling rates. ADC quantization noise is

discussed in the Applications section.

The preceding noise performance discussion applies to a

differential VGA output signal. Although the LNA noise

performance is the same in single-ended and differential

applications, the VGA performance is not. The noise of the

VGA is significantly higher in single-ended usage, since the

contribution of its bias noise is designed to cancel in the

differential signal. A transformer can be used with single-ended

applications when low noise is desired.

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