AD9670EBZ datasheet(23/53 Pages) AD

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AD9670

Data Sheet

Rev. A | Page 22 of 52

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 the

LO-x pin and the LI-x pin 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 postamplifier gain of 27 dB. These meas-

urements, 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 to 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 term-

ination, 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

Attach a 1 nF capacitor from CLNA (Ball B7) to AVDD2.

DC Offset Correction/High-Pass Filter

The AD9670 LNA architecture is designed to correct for dc offset

voltages that can develop on the external CS capacitor due to

leakage of the T/R 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.

TRANSDUCER

T/R

SWITCH CS

CSH

RFB1

RFB2

CLG

LO-x

LOSW-x

LI-x

LG-x

15.6dB,

17.9dB,

21.6dB

LNA

DC OFFSET

CORRECTION

AD9670

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 transconductance (gm) of the feedback

transconductance amplifier. The gm value is programmed in

operation.

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