THS4541IRUNR.B datasheet(21/66 Pages) TI2

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7 Parameter Measurement Information

7.1 Example Characterization Circuits

The THS4541 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. Figure 6-1 to Figure 6-36 shows the characterizations that focus on single-ended-to-differential

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

often simpler to 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. Figure 7-1 shows the starting

point for any single-ended-to-differential, ac-coupled characterization plot.

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

Figure 7-1. AC-Coupled, Single-Ended Source to a Differential Gain of a 2 V/V Test Circuit

Figure 7-1 shows how most characterization plots fix the Rf (Rf1 = Rf2) value at 402 Ω. 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. Figure 7-1 shows how the approximate total differential load 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 Section 7.5).

• 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.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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