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AD9271

Preliminary Technical Data

Rev. PrA | Page 22 of 58

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 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 3 can be used to calculate the needed RFB

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

)

(

(3)

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

resulting value is 190 Ω, resulting in a less than 0.1 dB gain

error. Factors such as 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, by the LNA BW. Furthermore, the input

capacitance and RS limit the BW at higher frequencies.

FREQUENCY (Hz)

100

100k

50M

10M

IN = 50Ω, RFB = 249Ω

SH = ∞, CSH = 0 pF

SH = 50Ω, CSH = 22 pF

IN = 100Ω, RFB = 499Ω

IN = 200Ω, RFB = 1kΩ

SH = ∞, CSH = 0 pF

SH = 50Ω, CSH = 22 pF

IN = 500Ω, RFB = 2.5kΩ

Figure 35. RIN vs. Frequency for Various Values of RFB

(Effects of RSH and CSH are Also Shown

Figure 35 shows RIN vs. frequency for various values of RFB. Note

that at the lowest value, 50 Ω, RIN peaks at frequencies greater

than 10 MHz. This is due to the BW roll-off of the LNA as

mentioned earlier.

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

6×

200

800

8×

200

1000

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 maximum

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

are taken without a feedback resistor, provide the basis for

calculating the input noise and noise figure performance of the

configurations shown in Figure 43. Figure 43 and Figure 44 are

simulations of noise figure vs. RS results using these

configurations and an input-referred noise voltage for the VGA

of 4 nV/√Hz. Unterminated (RFB = ∞) operation exhibits the

lowest equivalent input noise and noise figure. Figure 44 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 current noise.

VOUT

UNTERMINATED

–

VIN

RIN

VOUT

RESISTIVE TERMINATION

–

VIN

RIN

VOUT

ACTIVE IMPEDANCE MATCH

–

VIN

RIN

RFB

1 + A/2

RIN =

Figure 36. Input Configurations

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