THS4541_15 Datasheet, PDF (41/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

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THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
Transformer input coupling allows either a single-ended or differential source to be coupled into the THS4541;
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.
Rs
50
Pulse
CX2047LNL
1:2 Turns Ratio
Balun
C1
100 nF
M1
90.4 H
+ _VG1
±
N1 N2
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
402
Rg1
100
Vocm
Rg2
100
Vcc
±
+
FDA
±
+
PD
Vcc
R1
500
Output
Measurement
Point
Rf2
402
Figure 74. 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 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:
NF
§
¨
10
W
¨
Log_¨1

¨
(1  E ²)
E²

8
DE ²

4
(DE )²

§
¨©
e_ni
En
§1
W _{¨© 2}

1
D
· ^·2
¸¹ ¸¹

1
2
n
W
i_n W Rs²
E²
kTRs
·
¸
¸
¸
¸
¨©
¸¹
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)
(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 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).
PART
NUMBER
ADT2-1T
CX2047LNL
Rs (Ω)
50
50
Table 7. Example Input Step-Up Baluns and Associated Parameters
–1-dB
FREQUENCY
(MHz)
MIN
0.1
0.083
MAX
463
270
INSERTION
LOSS (dB)
0.3
0.2
MFR
MiniCircuits
Pulse Eng
NO. OF DECADES
–1-dB –3-dB
POINTS POINTS
3.67
4.22
3.51
3.93
–3-dB
FREQUENCY
(MHz)
MIN
0.05
0.044
MAX
825
372
TURNS
RATIO
1.41
2
MODEL ELEMENTS
L1 (µH)
79.57747
90.42894
L2 (µH)
158.50797
361.71578
k
0.99988
0.99976
M (µH)
112.19064
180.81512
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