AD9675KBCZ datasheet(24/61 Pages) AD

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Data Sheet

AD9675

Rev. A | Page 23 of 60

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 impor-

tant 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 configura-

tions, respectively. The noise figures for 200 Ω are 4.5 dB, 1.7 dB,

and 1 dB, respectively.

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.

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 AD9675 LNA architecture corrects for dc offset voltages

that can develop on the external CS capacitor due to leakage of

the transmit (Tx)/receive (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.

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 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.

Table 10. High-Pass Filter Cutoff Frequency, fHP, for CLG = 10 nF

Address

0x120[4:3]

LNAGAIN =

15.6 dB

LNAGAIN =

17.9 dB

LNAGAIN =

21.6 dB

00 (default)

0.5 mS

41 kHz

55 kHz

83 kHz

1.0 mS

83 kHz

110 kHz

167 kHz

1.5 mS

133 kHz

178 kHz

267 kHz

2.0 mS

167 kHz

220 kHz

330 kHz

100

1.5

3.0

4.5

6.0

7.5

9.0

10.5

12.0

RS (Ω)

UNTERMINATED

ACTIVE TERMINATION

SHUNT TERMINATION

100

RS (Ω)

RIN = 50Ω

RIN = 75Ω

RIN = 100Ω

RIN = 200Ω

UNTERMINATED

LNA

LOSW-x

LO-x

LI-x

LG-x

RFB2

RFB1

CLG

CSH

TRANSDUCER

Tx/Rx

SWITCH

15.6dB,

17.9dB,

21.6dB

AD9675

DC OFFSET

CORRECTION

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