THS4541_16 Datasheet, PDF (21/61 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
8 Parameter Measurement Information
8.1 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. 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.
50- Input Match,
Gain of 2 V/V from Rt,
Single-Ended Source to
Differential Output
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
402
50- 
Source
C1
Rg1
100 nF ₁₉₁
Rt
60.2
Vocm
Rg2
221
Vcc
±
+
FDA
±
+
PD
Vcc
Rload
500
Output
Measurement
Point
C2
100 nF
Rf2
402
Figure 61. AC-Coupled, Single-Ended Source to a Differential Gain of a 2-V/V Test Circuit
Most characterization plots fix the R_f (R_f1 = R_f2) 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.
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