THS4541 datasheet(41/64 Pages) TI1

41 / 64 page

‡

(

)

‡

(

)

n i

10 Log 1

k Rs

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

THS4541-Q1

www.ti.com

SLOS930B – NOVEMBER 2015 – REVISED NOVEMBER 2019

Product Folder Links: THS4541-Q1

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Transformer input coupling allows either a single-ended or differential source to be coupled into the THS4541-

Q1; possibly also improving the input-referred noise figure. These designs assume a source impedance that

must be matched in the balun interface. The simplest approach is shown in Figure 74, where an example 1:2

turns ratio step-up transformer is used from a 50-Ω source.

Figure 74. Input Balun Interface Delivers a Differential Input to the THS4541-Q1

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 they 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:

where

•

≡ turns ratio (the ohms ratio is then n

•

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)

(14)

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 74) 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-Q1 holds exceptional SFDR for less than 40-MHz applications.

Two representative selections, with their typical measured spans and resulting model elements, are shown in

Table 7. 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 7. 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

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