THS4541 datasheet(21/64 Pages) TI1

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100 nF

Rf1

402

Rload

500

Rg1

191

100 nF

Rg2

221

60.2

Output

Measurement

Point

THS4541 Wideband,

Fully-Differential Amplifier

50-

Input Match,

Gain of 2 V/V from Rt,

Single-Ended Source to

Differential Output

Vocm

Rf2

402

50-

Source

Vcc

Vocm

Vcc

FDA

THS4541-Q1

www.ti.com

SLOS930B – NOVEMBER 2015 – REVISED NOVEMBER 2019

Product Folder Links: THS4541-Q1

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8 Parameter Measurement Information

8.1 Example Characterization Circuits

The THS4541-Q1 offers the advantages of a fully differential amplifier (FDA) design, with the trimmed input offset

voltage of a precision op amp. The FDA is an extremely flexible device that provides a purely differential output

signal centered on a settable output common-mode level. The primary options revolve around the choices of

single-ended or differential inputs, AC-coupled or DC-coupled signal paths, gain targets, and resistor-value

selections. The characterizations shown in Figure 1 to Figure 36 focus on single-ended-to-differential designs as

the more challenging application requirement. Differential sources can certainly be supported and are often

simpler to both implement and analyze.

Because most lab equipment is single-ended, the characterization circuits typically operate with a single-ended,

matched, 50-Ω input termination to a differential output at the FDA output pins. That output is then translated

back to single-ended through a variety of baluns (or transformers) depending on the test and frequency range.

DC-coupled, step-response testing uses two 50-Ω scope inputs with trace math. The starting point for any single-

ended-to-differential, AC-coupled characterization plot is shown in Figure 61.

Figure 61. AC-Coupled, Single-Ended Source to a Differential Gain of a 2-V/V Test Circuit

Most characterization plots fix the Rf (Rf1 = Rf2) value at 402 Ω, as shown in Figure 61. This element value is

completely flexible in application, but the 402 Ω provides a good compromise for the parasitic issues linked to

this value, specifically:

•

Added output loading. The FDA appears like an inverting op amp design with both feedback resistors as an

added load across the outputs (approximate total differential load in Figure 61 is 500 Ω || 804 Ω = 308 Ω).

•

Noise contributions because of the resistor values. The resistors contribute both a 4kTR term and provide

gain for the input current noise (see the Noise Analysis section).

•

Parasitic feedback pole at the input summing nodes. This pole created by the feedback R value and the

0.85-pF differential input capacitance (as well as any board layout parasitic) introduces a zero in the noise

gain, decreasing the phase margin in most situations. This effect must be managed for best frequency

response flatness or step response overshoot. The 402-Ω value selected does degrade the phase margin

slightly over a lower value, but does not decrease the loading significantly from the nominal 500-Ω value

across the output pins.

THS4541 Datasheet(PDF) 21 Page - Texas Instruments

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