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8 Parameter Measurement Information

8.1 Example Characterization Circuits

The THS4561 offers the advantages of a fully differential amplifier (FDA) design with the trimmed input offset

voltage and low drift of a precision op amp. The FDA is a flexible device where the main aim is to provide a

purely differential output signal centered on a user-configurable common-mode voltage usually matched to the

input common-mode voltage required by an analog-to-digital converter (ADC) following the FDA stage. 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 characterization circuits described in this section

focus on single-ended input to differential output designs as the more challenging application requirement.

Differential sources are supported and are simple to implement and analyze.

The characterization circuits are typically operated with a single-ended, matched, 50-Ω, input termination to a

differential output at the FDA output pins because most lab equipment is single-ended. The FDA differential

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 to

measure the differential output. Single-supply operation is most common in end equipment designs. However,

using split balanced supplies allows simple ground referenced testing without adding further blocking capacitors

in the signal path beyond those capacitors already within the test equipment. The starting point for any single-

ended input to differential output measurements (such as any of the frequency response curves) is shown in

Figure 8-1.

1.5 k

52.3

31.8-dB

Insertion Loss

from VOPP to a

50-

Load

THS4561 Wideband,

Fully Differential Amplifier

50-

Input Match,

Gain of 1 V/V from RT,

Single-Ended Source to

Differential Output

VOCM

1.5 k

Network

Analyzer,

50-

Source

Impedance

S+

FDA

S±

52.3

50-

Termination

487

52.3

Network

Analyzer,

50-

Load

ADTL1-4-75+

1-k

Differential

Load

50-

Single-Ended

Source

Figure 8-1. Single-Ended Source to a Differential Gain of a 1-V/V Test Circuit

Most characterization plots fix the RF (RF1 = RF2) value at 1.5 kΩ, as shown in Figure 8-1. This element value is

flexible in application, but 1.5 kΩ provides a good compromise for the parasitic issues linked to this value,

specifically:

• Added output loading: The FDA functions similarly to an inverting op amp design with feedback resistors

appearing as an added load across the outputs (the approximate total differential load in Figure 8-1 is 1.5 kΩ

|| 1 kΩ = 857 Ω). The 1.5-kΩ value reduces the power dissipated in the feedback networks.

• Noise contributions resulting from resistor values. These contributions are both the 4kTRF terms and the

current noise times the RF term referred to the output (see Section 10.1.3).

• Parasitic feedback pole at the input summing nodes. This pole is created by the feedback resistor (RF) value

and the 2.4-pF differential input capacitance (as well as any board layout parasitic) and introduces a zero in

the noise gain, which decreases the phase margin in most situations. This effect must be managed for best

frequency response flatness or step response overshoot.

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THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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