THS4521 datasheet(20/51 Pages) TI

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The frequency domain characterization curves start with the circuit and component selections of Figure 8-1.

Some of the features in this test circuit include:

• The elements on the non-signal input side match the signal input resistors. This feature closely matches the

divider networks on each side of the FDA. The three resistors (RG2, RT2, and RS1) on the non-signal input

side can be replaced by a single resistor to ground using a standard value of 1.5 kΩ with some loss in gain

balancing between the two sides.

• Translating from a 1-kΩ differential load to a 50-Ω environment introduces considerable insertion loss in the

measurements (–31.8 dB in Figure 8-1). The measurement path insertion loss normalizes when reporting the

frequency response curves to show the gain response to the FDA output pins.

• In the pass band for the output balun, the 50-Ω load of the network analyzer reflects in parallel with the 52.3-

Ω shunt termination, RM. These elements combine to show a differential 1-kΩ load at the output pins of the

THS4561. The source impedance presented to the balun is a differential 50-Ω source. Figure 8-2 and Figure

8-3 show the TINA-TI™ model (available as a TINA-TI™ simulation file) and resulting response flatness for

this relatively low-frequency balun providing 0.1-dB flatness through 100 MHz.

ADTL1-4-75 Model 198.94µ

VM1

VG1

L1 Inductance: 198.94 µH

L2 Inductance: 198.94 µH

Mutual Inductance: 198.92972 µH

Figure 8-2. Output Measurement Balun Simulation Circuit in TINA-TI™

Frequency (Hz)

-6.1

-10

-6.09

-8

-6.08

-6

-6.07

-4

-6.06

-2

-6.05

-6.04

-6.03

-6.02

-6.01

-6

10k

100k

10M

100M

D061

Gain (dB)

Phase (deg)

Figure 8-3. Output Measurement Balun Flatness Test

THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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