AD9671EBZ datasheet(24/61 Pages) AD

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Data Sheet

AD9671

Rev. A| Page 23 of 60

Table 9 lists the recommended values for RFB and CSH in terms

of RIN. CFB is needed in series with RFB because the dc levels at

Pin LO-x and Pin LI-x are unequal.

Table 9. Active Termination External Component Values

LNA Gain (dB)

RIN (Ω)

RFB (Ω)

Minimum CSH (pF)

15.6

150

17.9

200

21.6

300

15.6

100

350

17.9

100

450

21.6

100

650

15.6

200

750

Not applicable

17.9

200

950

Not applicable

21.6

200

1350

Not applicable

LNA Noise

The short-circuit noise voltage (input referred noise) is an important

limit on system performance. The short-circuit noise voltage for

the LNA is 0.78 nV/√Hz at a gain of 21.6 dB, including the VGA

noise at a VGA postamp gain of 27 dB. These measurements,

which were taken without a feedback resistor, provide the basis for

calculating the input noise and noise figure (NF) performance.

Figure 35 and Figure 36 are simulations of noise figure vs. RS results

with different input configurations and an input referred noise

voltage of 2.5 nV/√Hz for the VGA. Unterminated (RFB = ∞)

operation exhibits the lowest equivalent input noise and noise

figure. Figure 36 shows the noise figure vs. source resistance rising

at low RS, where the LNA voltage noise is large compared with the

source noise, and at high RS due to the noise contribution from RFB.

The lowest NF is achieved when RS matches RIN.

Figure 35 shows the relative noise figure performance. With an LNA

gain of 21.6 dB, the input impedance is swept with RS to preserve

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

of 50 Ω are 7 dB, 4 dB, and 2.5 dB for the shunt termination,

active termination, and unterminated configurations, respectively.

The noise figures for 200 Ω are 4.5 dB, 1.7 dB, and 1 dB,

respectively.

100

1.5

3.0

4.5

6.0

7.5

9.0

10.5

12.0

RS (Ω)

UNTERMINATED

ACTIVE TERMINATION

SHUNT TERMINATION

Figure 35. Noise Figure vs. RS for Shunt Termination, Active

Termination Matched and Unterminated Inputs, VGAIN = 1.6 V

Figure 36 shows the noise figure as it relates to RS for various

values of RIN, which is helpful for design purposes.

100

RS (Ω)

RIN = 50Ω

RIN = 75Ω

RIN = 100Ω

RIN = 200Ω

UNTERMINATED

Figure 36. Noise Figure vs. RS for Various Fixed Values of RIN,

Active Termination Matched Inputs, VGAIN = 1.6 V

CLNA Connection

CLNA (Pin B7) must have a 1 nF capacitor attached to AVDD2.

DC Offset Correction/High-Pass Filter

The AD9671 LNA architecture is designed to correct for dc offset

voltages that can develop on the external CS capacitor due to

leakage of the Tx/Rx switch during ultrasound transmit cycles.

The dc offset correction, as shown in Figure 37, provides a

feedback mechanism to the LG-x input of the LNA to correct

for this dc voltage.

LNA

LOSW-x

LO-x

LI-x

LG-x

RFB2

RFB1

CLG

CSH

TRANSDUCER

Tx/Rx

SWITCH

15.6dB,

17.9dB,

21.6dB

AD9671

DC OFFSET

CORRECTION

Figure 37. Simplified LNA Input Configuration

The feedback acts as high-pass filter providing dynamic

correction of the dc offset. The cutoff frequency of the high-

pass filter response is dependent on the value of the CLG capacitor,

the gain of the LNA (LNAGAIN) and the trandsconductance ( gm) of

the feedback transconductance amplifier. The gm value is

programmed in Register 0x120, Bits[4:3]. Ensure that CS is

equal to CLG for proper operation.

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