AD8332ACP-R2 datasheet(27/40 Pages) AD

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AD8331/AD8332/AD8334

Rev. E | Page 27 of 40

The primary purpose of input impedance matching is to

improve the system transient response. With resistive termination,

the input noise increases due to the thermal noise of the

matching resistor and the increased contribution of the LNA’s

input voltage noise generator. With active impedance matching,

however, the contributions of both are smaller than they would

be for resistive termination by a factor of 1/(1 + LNA Gain).

Figure 76 shows their relative noise figure (NF) performance. In

this graph, the input impedance was swept with RS to preserve

the match at each point. The noise figures for a source impedance

of 50 Ω are 7.1 dB, 4.1 dB, and 2.5 dB, respectively, for the

resistive, active, and unterminated configurations. The noise

figures for 200 Ω are 4.6 dB, 2.0 dB, and 1.0 dB, respectively.

Figure 77 is a plot of the NF vs. RS for various values of RIN,

which is helpful for design purposes. The plateau in the NF for

actively matched inputs mitigates source impedance variations.

For comparison purposes, a preamp with a gain of 19 dB and

noise spectral density of 1.0 nV/√Hz, combined with a VGA

with 3.75 nV/√Hz, yields a noise figure degradation of

approximately 1.5 dB (for most input impedances), significantly

worse than the AD8332 performance.

The equivalent input noise of the LNA is the same for single-

ended and differential output applications. The LNA noise figure

improves to 3.5 dB at 50 Ω without VGA noise, but this is

exclusive of noise contributions from other external circuits

connected to LOP. A series output resistor is usually

recommended for stability purposes when driving external

circuits on a separate board (see the Applications section). In

low noise applications, a ferrite bead is even more desirable.

VARIABLE GAIN AMPLIFIER

The differential X-AMP VGA provides precise input

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

of 2.7 nV/√Hz and excellent gain linearity. A simplified block

diagram is shown in Figure 78.

VIP

GAIN

6dB

48dB

VIN

POSTAMP

–

GAIN INTERPOLATOR

(BOTH CHANNELS)

Figure 78. Simplified VGA Schematic

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 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 gain-of-12 feedback amplifier

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

stage is designed to reduce feedthrough to the output and to

ensure excellent frequency response uniformity across gain

setting (see Figure 12 and Figure 13).

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

Equation 1 and Equation 2.

Gain accuracy is very good because 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 (Figure 11

shows gain errors in the center of the control range). When

VGAIN < 0.1 or > 0.95, gain errors are slightly greater.

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