THS4541IRUNR.B datasheet(43/66 Pages) TI2

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Using the typical input referred noise terms for the THS4541 (eni = 2.2 nV and in = 1.9 pA) and sweeping the

total gain from the input of the balun to the differential output over a 10-dB to 24-dB span, gives the input noise

figure shown in Figure 8-4.

Total Gain (dB)

D061

ADT2-1T

CX2047LNL

Figure 8-4. Noise Figure versus Total Gain with the Two Input Baluns of Table 8-2

The 50-Ω referred noise figure estimates show a decreasing input-referred noise for either balun as the gain

increases through 24 dB. To achieve the total target gain after the step up from the input balun, the only

elements changing in these sweeps are the feedback-resistor values. The example of Figure 8-3 is a gain of

7.86 V/V, or a 17.9-dB gain where a 9.0-dB input noise figure is predicted from Figure 8-4. Another advantage

for this method is that the effective noise gain (NG) is reduced by the source impedance appearing as part of the

total Rg element in the design. The example of Figure 8-3 operates with a NG = 1 + 402 / (100 + 100) = 3 V/V,

giving greater than 300-MHz SSBW in the THS4541 portion of the design. Combining that capability with the 372

MHz in the balun gives greater than 200 MHz in this 18-dB gain stage; or an equivalent greater than 1.6-GHz

gain bandwidth product in a low-power, high dynamic range interface.

Added features and considerations for the balun input of Figure 8-3 include:

• Many of these baluns offer a secondary centertap. Leave the centertap unconnected for the best HD2

suppression and DC biasing (do not include a capacitor from this centertap to ground).

• With a floating secondary centertap, the input pins common-mode voltage again equals the output Vocm

setting because there is no DC path for the output common-mode voltage to create a common-mode current

(ICM).

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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THS4541IRUNR.B Datasheet(PDF) 43 Page - Texas Instruments