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AD8331/AD8332/AD8334

Rev. E | Page 26 of 40

Active Impedance Matching

The LNA supports active impedance matching through an external

shunt feedback resistor from Pin LON to Pin INH. The input

resistance, RIN, is given by Equation 5, where A is the single-

ended gain of 4.5, and 6 kΩ is the unterminated input impedance.

kΩ

(5)

CFB is needed in series with RFB because the dc levels at Pin LON

and Pin INH are unequal. Expressions for choosing RFB in terms

of RIN and for choosing CFB are found in the Applications section.

CSH and the ferrite bead enhance stability at higher frequencies

where the loop gain is diminished and prevent peaking. Frequency

response plots of the LNA are shown in Figure 23 and Figure 24.

The bandwidth is approximately 130 MHz for matched input

impedances of 50 Ω to 200 Ω and declines at higher source

impedances. The unterminated bandwidth (when RFB = ∞) is

approximately 80 MHz.

Each output can drive external loads as low as 100 Ω in addition

to the 100 Ω input impedance of the VGA (200 Ω differential).

Capacitive loading up to 10 pF is permissible. All loads should

be ac-coupled. Typically, Pin LOP output is used as a single-

ended driver for auxiliary circuits, such as those used for

Doppler ultrasound imaging, and Pin LON drives RFB.

Alternatively, a differential external circuit can be driven from

the two outputs in addition to the active feedback termination.

In both cases, important stability considerations discussed in

the Applications section should be carefully observed.

The impedance at each LNA output is 5 Ω. A 0.4 dB reduction

in open-circuit gain results when driving the VGA, and 0.8 dB

with an additional 100 Ω load at the output. The differential

gain of the LNA is 6 dB higher. If the load is less than 200 Ω on

either side, a compensating load is recommended on the

opposite output.

LNA Noise

The input-referred voltage noise sets an important limit on

system performance. The short-circuit input voltage noise of

the LNA is 0.74 nV/√Hz or 0.82 nV/√Hz (at maximum gain),

including the VGA noise. The open-circuit current noise is

2.5 pA/√Hz. These measurements, taken without a feedback

resistor, provide the basis for calculating the input noise and

noise figure performance of the configurations in Figure 75.

Figure 76 and Figure 77 are simulations extracted from these

results, and the 4.1 dB NF measurement with the input actively

matched to a 50 Ω source. Unterminated (RFB = ∞) operation

exhibits the lowest equivalent input noise and noise figure.

Figure 76 shows the noise figure vs. source resistance, rising at

low RS, where the LNA voltage noise is large compared to the

source noise, and again at high RS due to current noise. The

VGA’s input-referred voltage noise of 2.7 nV/√Hz is included in

all of the curves.

VOUT

UNTERMINATED

–

VIN

RIN

VOUT

RESISTIVE TERMINATION

–

VIN

RIN

VOUT

ACTIVE IMPEDANCE MATCH

–

VIN

RIN

RFB

1 + 4.5

RIN =

Figure 75. Input Configurations

100

RS (Ω)

INCLUDES NOISE OF VGA

RESISTIVE TERMINATION

(RS = RIN)

ACTIVE IMPEDANCE MATCH

UNTERMINATED

SIMULATION

Figure 76. Noise Figure vs. RS for Resistive,

Active Matched and Unterminated Inputs

100

RS (Ω)

INCLUDES NOISE OF VGA

RIN = 50Ω

RIN = 75Ω

RIN = 100Ω

RIN = 200Ω

RFB = ∞

SIMULATION

Figure 77. Noise Figure vs. RS for Various Fixed Values of RIN, Actively Matched

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