THS4521 datasheet(23/51 Pages) TI

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8.7 Terminology and Application Assumptions

There are common terms that are unique to this device. This section identifies and explains these terms.

• Fully differential amplifier (FDA). This term is restricted to devices offering what is similar to a differential

inverting op amp design element that requires an input resistor (not a high-impedance input) and includes a

second internal control loop that sets the output average voltage (VOCM) to a default or set point. This second

common-mode control loop interacts with the differential loop in certain configurations.

• The desired output signal at the two output pins is a differential signal that swings symmetrically around a

common-mode voltage, VOCM, which is the average voltage of the two outputs.

• Single-ended to differential. Generally the output is always used differentially in an FDA; however, the source

signal can be either a single-ended or a differential source. For an FDA operating in single-ended to

differential, the input is applied only to one of the two inputs via input resistors.

• The common-mode control loop has limited bandwidth from the input VOCM pin to the common-mode output

voltage. The internal loop bandwidth beyond the input VOCM buffer is a much wider bandwidth than the

reported VOCM bandwidth, but is not directly discernable. A very wide bandwidth in the internal VOCM loop

is required to perform an effective and low-distortion single-ended to differential conversion.

Several features in the application of the THS4561 are not explicitly stated, but are necessary for correct

operation. These features are:

• Although often not stated, the disable pin ( PD) is tied to the positive supply when an enabled channel is

desired.

• Virtually all ac characterization equipment expects a 50-Ω termination from the 50-Ω source and a 50-Ω,

single-ended source impedance from the device outputs to the 50-Ω sensing termination. This condition is

achieved in all characterizations (often with some insertion loss) but is not necessary for most applications.

Matching impedance is most often required when transmitting over longer distances. Tight layouts from a

source, through the THS4561, and to an ADC input do not require doubly-terminated lines or filter designs.

The only exception is if the source requires a defined termination impedance for correct operation (for

example, mixer outputs).

• The amplifier signal path is flexible for use as single-supply or split-supply operation. Most applications are

intended to be single supply, but any split-supply design can be used as long as the total supply voltage

across the TH4561 is less than 12.6 V and the required input, output, and common-mode pin headrooms to

each supply are taken into account. When left open, the VOCM pin defaults to near midsupply for any

combination of split or single supplies used.

• External element values are normally assumed to be accurate and matched. In an FDA, this assumption

translates to equal feedback resistor values and a matched impedance from each input summing junction to

either a signal source or a DC bias reference on each side of the inputs. Unbalancing these values introduces

non-idealities in the signal path. For the signal path, imbalanced resistor ratios on the two sides creates a

common-mode to differential conversion. Furthermore, mismatched RF values and feedback ratios create

additional differential output error terms from any common-mode DC or AC signal or noise terms. Using

standard 1% resistor values is a typical approach and generally leads to some nominal feedback ratio

mismatch. Modestly mismatched resistors or ratios do not by themselves degrade harmonic distortion. Where

there is a meaningful common-mode noise or distortion in the input signal, that gets converted to differential

via an element or ratio mismatch. For the best DC precision, use 0.1% accuracy resistors that are readily

available in E96 values.

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THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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