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THS4541 Datasheet, PDF (42/62 Pages) Texas Instruments – Fully Differential Amplifier
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THS4541
SLOS375 – AUGUST 2014
www.ti.com
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 75.
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ADT2-1T
13
CX2047LNL
12
11
10
9
8
7
10
12
14
16
18
20
22
24
Total Gain (dB)
D061
Figure 75. Noise Figure versus Total Gain with the Two Input Baluns of Table 7
The 50-Ω referred noise figure estimates show a decreasing input-referred noise for either balun as the gain
increases through 24 dB. The only element changing in these sweeps are the feedback-resistor values, in order
to achieve the total target gain after the step up from the input balun. The example of Figure 74 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 75. 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 74 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 with the 372 MHz in
the balun itself 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 74 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 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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