AD8331 datasheet(19/32 Pages) AD

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AD8331/AD8332

Rev. C | Page 19 of 32

VOUT

UNTERMINATED

–

VIN

RIN

VOUT

RESISTIVE TERMINATION

–

VIN

RIN

VOUT

ACTIVE IMPEDANCE MATCH –RS = RIN

–

VIN

RIN

RFB

1 + 4.5

RIN =

Figure 60. Input Configurations

100

RS (Ω)

ACTIVE IMPEDANCE MATCH

RESISTIVE TERMINATION

(RS = RIN)

UNTERMINATED

SIMULATION

INCLUDES NOISE OF VGA

Figure 61. Noise Figure vs. RS for Resistive,

Active Matched, and Unterminated Inputs

100

RIN = 50Ω

Ω

RFB = ∞

RS (Ω)

200

Ω

INCLUDES NOISE OF VGA

100

Ω

SIMULATION

Figure 62. Noise Figure vs. RS for Various Fixed Values of RIN, Actively Matched

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 61 shows their relative noise figure (NF)

performance. In this graph, the input impedance has been 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 62 is a plot of the NF versus 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 a 1.0 nV/√Hz, combined with a VGA

with 3.75 nV/√Hz, would yield 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 63.

GAIN INTERPOLATOR

(BOTH CHANNELS)

POST-AMP

VIP

GAIN

6dB

48dB

VIN

POST-AMP

Figure 63. Simplified VGA Schematic

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