THS4521 datasheet(42/66 Pages) TI2

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Rf1

402

500

Rg1

100

Rg2

100

Output

Measurement

Point

THS4541 Wideband,

Fully-Differential Amplifier

Rf2

402

Pulse

CX2047LNL

1:2 Turns Ratio

Balun

VG1

90.4 H

100 nF

Vcc

Vocm

Vcc

FDA

Figure 8-3. Input Balun Interface Delivers a Differential Input to the THS4541

In this example, this 1:2 turns ratio step-up transformer provides a source and load match from the 50-Ω source

if the secondary is terminated in 200 Ω (turns-ratio squared is the impedance ratio across a balun). The two Rg

elements provide that termination as the elements sum to the differential virtual ground at the FDA summing

junctions. The input blocking cap (C1) is optional and included only to eliminate dc shorts to ground from the

source. This solution often improves the input-referred noise figure more so than just the FDA using this passive

(zero power dissipation) input balun. Defining a few ratios allows a noise figure expression to be written as

Equation 14:

‡

(

)

‡

(

)

n i

10 Log 1

k Rs

(14)

where

• n ≡ turns ratio (the ohms ratio is then n2)

• α ≡ differential gain in the FDA = Rf / Rg

• β ≡ transformer insertion loss in V/V (from a dB insertion loss, convert to linear attenuation = β)

• kT = 4e-21J at 290 K (17°C)

One way to use Equation 14 is to fix the input balun selection, and then sweep the FDA gain by stepping up

the Rf value. The lowest-noise method uses just the two Rg elements for termination matching (no Rm element,

such as in Figure 8-3) and sweep the Rf values up to assess the resulting input-referred noise figure. While this

method can be used with all FDAs and a wide range of input baluns, relatively low-frequency input baluns are an

appropriate choice here because the THS4541 holds exceptional SFDR for less than 40-MHz applications. Two

representative selections, with typical measured spans and resulting model elements, are shown in Table 8-2.

For these two selections, the critical inputs for the noise figures are the turns ratio and the insertion loss (the 0.2

dB for the CX2014LNL becomes a β = 0.977 in the NF expression).

Table 8-2. Example Input Step-Up Baluns and Associated Parameters

PART

NUMBER

Rs (Ω)

–1-dB FREQUENCY

(MHz)

INSERTION

LOSS (dB)

MFR

NO. OF DECADES

–3-dB FREQUENCY

(MHz)

TURNS

RATIO

MODEL ELEMENTS

MIN

MAX

–1-dB

POINTS

–3-dB

POINTS

MIN

MAX

L1 (µH)

L2 (µH)

M (µH)

ADT2-1T

0.1

463

0.3

MiniCircuits

3.67

4.22

0.05

825

1.41

79.57747

158.50797

0.99988

112.19064

CX2047LNL

0.083

270

0.2

Pulse Eng

3.51

3.93

0.044

372

90.42894

361.71578

0.99976

180.81512

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

www.ti.com

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