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AD9271

Rev. B | Page 22 of

Because the amplifier has a gain of 6× from its input to its

differential output, it is important to note that the gain A/2 is

the gain from Pin LI-x to Pin LO-x, and it is 6 dB less than the

gain of the amplifier, or 9.6 dB (3×). The input resistance is

reduced by an internal bias resistor of 15 kΩ in parallel with the

source resistance connected to Pin LI-x, with Pin LG-x ac

grounded. Equation 2 can be used to calculate the needed RFB

for a desired RIN, even for higher values of RIN.

)

(

(2)

For example, to set RIN to 200 Ω, the value of RFB is 845 Ω. If the

simplified equation (Equation 2) is used to calculate RIN, the

value is 190 Ω, resulting in a gain error less than 0.5 dB. Some

factors, such as the presence of a dynamic source resistance,

might influence the absolute gain accuracy more significantly.

At higher frequencies, the input capacitance of the LNA needs

to be considered. The user must determine the level of

matching accuracy and adjust RFB accordingly.

The bandwidth (BW) of the LNA is about 70 MHz. Ultimately

the BW of the LNA limits the accuracy of the synthesized RIN.

For RIN = RS up to about 200 Ω, the best match is between

100 kHz and 10 MHz, where the lower frequency limit is

determined by the size of the ac-coupling capacitors, and the

upper limit is determined by the LNA BW. Furthermore, the

input capacitance and RS limit the BW at higher frequencies.

Figure 42 shows RIN vs. frequency for various values of RFB.

100k

10M

50M

FREQUENCY (Hz)

100

RS = 50Ω, RFB = 200Ω, CSH = 70pF

RS = 100Ω, RFB = 400Ω, CSH = 20pF

RS = 200Ω, RFB = 800Ω

RS = 500Ω, RFB = 2kΩ

Figure 42. RIN vs. Frequency for Various Values of RFB

(Effects of RSH and CSH Are Also Shown)

Note that at the lowest value, 50 Ω, in Figure 42, RIN peaks at

frequencies greater than 10 MHz. This is due to the BW roll-off

of the LNA, as mentioned previously.

However, as can be seen for larger RIN values, parasitic capacitance

starts rolling off the signal BW before the LNA can produce

peaking. CSH further degrades the match; therefore, CSH should

not be used for values of RIN that are greater than 100 Ω. Table 7

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 7. Active Termination External Component Values

LNA Gain

RIN (Ω)

RFB (Ω)

Minimum

CSH (pF)

BW (MHz)

5×

175

6×

200

8×

250

5×

100

350

6×

100

400

8×

100

500

5×

200

700

N/A

6×

200

800

N/A

8×

200

1000

N/A

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 1.2 nV/√Hz or 1.4 nV/√Hz (at 15.6 dB

LNA gain), including the VGA noise. These measurements,

which were taken without a feedback resistor, provide the basis

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

of the configurations shown in Figure 43. Figure 44 and Figure 45

are simulations of noise figure vs. RS results using these config-

urations and an input-referred noise voltage of 4 nV/√Hz for

the VGA. Unterminated (RFB = ∞) operation exhibits the lowest

equivalent input noise and noise figure. Figure 45 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.

VOUT

UNTERMINATED

–

VIN

RIN

VOUT

RESISTIVE TERMINATION

–

VIN

RIN

VOUT

ACTIVE IMPEDANCE MATCH

–

VIN

RIN

RFB

1 + A/2

RIN =

Figure 43. Input Configurations

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