Distinguish differential transfer from control of the output average
The THS4551 terminology describes two controlled quantities at its outputs. Their difference carries the desired signal, while their average is the output common-mode voltage. The device includes a differential signal loop and a second internal loop that controls this average. Choosing the average places the output pair within its voltage environment; choosing the differential transfer describes how the input signal appears between those outputs.
The desired outputs move symmetrically around the common-mode level. This gives the two descriptions a shared physical location without making them interchangeable. Changing the target average concerns the position around which the pair operates. The differential signal remains the relationship between the outputs. A common-mode target alone therefore supplies neither the intended signal gain nor the full voltage excursion of either output.
A single-ended source does not change that output definition. TI describes single-ended-to-differential operation as applying the source signal on one input side while using the FDA output differentially. The source format and the output format are separate architectural facts. This is why a sourcing requirement for the named FDA must preserve both its differential signal function and its output-average control, rather than reducing them to one output-bias label.
Separate external common-mode response from internal loop action
The datasheet distinguishes the response from the VOCM control input from the action of the internal common-mode loop. The path from the control pin to the output average has limited bandwidth. Beyond the input buffer, the internal loop has a much wider bandwidth that is not directly discernible from that control-pin response. The two bandwidth descriptions refer to different portions of the control architecture.
TI connects the wide internal-loop response with effective, low-distortion conversion from a single-ended source to differential outputs. During that operation, internal common-mode control has a role beyond following deliberate changes applied at VOCM. A control-input response figure therefore cannot describe every common-mode action occurring as the differential signal path operates. Its measurement path must remain attached to its meaning.
This also separates common-mode control from a general differential-bandwidth headline. The differential transfer concerns the signal between outputs; the external VOCM path concerns movement of their commanded average; the internal loop supports the device's common-mode behavior within the conversion process. Keeping those paths distinct makes a bandwidth comparison intelligible without treating one quoted response as a complete description of the amplifier's two interacting control loops.
Keep resistor-ratio balance in the common-to-differential error path
The FDA's external network participates in converting common-mode quantities into differential output error. TI assumes equal feedback elements and matched impedances on the two input sides when describing the intended arrangement. When the feedback values or the gain ratios differ, terms that would otherwise have a common-mode relationship can appear as a difference between the outputs. The surrounding resistor network is therefore part of the error path.
Absolute feedback-resistor mismatch and mismatch between the two gain ratios have distinguishable roles. The datasheet associates feedback-value imbalance with an input-bias-current contribution, while unequal ratios allow common-mode terms to become differential error. Its named examples of those terms include source common-mode voltage, the desired output common-mode level and drift in the VOCM reference or control path. Merely quoting one resistor's nominal accuracy leaves the relationship between the two sides unspecified.
Control of the output average continues within this description. The common-mode loop can hold that average while an imbalance produces differential error around it. A correctly positioned pair can consequently have an additional unwanted difference. The output-common-mode target and resistor-ratio balance answer different precision questions, even though the resulting errors arise within the same amplifier and external feedback arrangement.
Check individual output swing and moving input common mode
An output-average target occupies part of the available voltage environment. Each output must still move around that target while respecting its own permitted swing and supply headroom. A differential-swing requirement describes the pair together, whereas clipping occurs at an individual output. TI's qualitative discussion therefore considers the common-mode level and the excursions on both sides as connected conditions of the intended output.
The input side has another common-mode relationship. In single-ended-to-differential operation, the amplifier's input-pin common-mode voltage can move with the applied signal. TI explains that this motion changes the apparent input impedance from the simple value of the input resistor alone. The active input relationship applies to both ac-coupled and dc-coupled designs in the named discussion. It is part of how the source interacts with the FDA.
These input and output conditions cannot be resolved by selecting only the desired output average. That target establishes the pair's center, while individual output excursions require headroom and the moving input common mode affects the source interface. Reading all three relationships together preserves the architecture's voltage constraints without transferring a general supply-range or rail-to-rail label into a conclusion about the complete signal excursion.
Sources and further reading
THS4551 SBOS778D bounded terminology, imbalance and operating-mode prose on physical pages 27, 35, 36 and 38. No equation, plot value, resistor calculation or achieved distortion result is adopted.