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AD8331/AD8332/AD8334

Rev. G | Page 26 of 56

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 in Equation 5, where A is the single-

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

kΩ

(5)

CIZ is needed in series with RIZ because the dc levels at Pin LON

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

of RIN and for choosing CIZ are found in the Applications

Information 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 RIZ = ∞) 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. Pin LON drives RIZ. 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

Information 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 a 0.8 dB

reduction results 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 show simulations extracted from these

results and the 4.1 dB noise figure (NF) measurement with the

input actively matched to a 50 Ω source. Unterminated (RIZ = ∞)

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 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 - RS = RIN

–

VIN

RIN

RIZ

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 Match, and Unterminated Inputs

100

RS (Ω)

INCLUDES NOISE OF VGA

RIN = 50Ω

RIN = 75Ω

RIN = 100Ω

RIN = 200Ω

RIZ = ∞

(SIMULATED RESULTS)

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

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