Like all widely-used devices, numerous common terms have developed that are unique to this type of device. These terms include:

• Fully differential amplifier (FDA)—In this document, this term is restricted to devices offering what appears similar to a differential inverting op amp design element that requires an input resistor (not high-impedance input) and includes a second internal control-loop setting the output average voltage (Vocm) to a default or set point. This second loop interacts with the differential loop in some configurations.
• The desired output signal at the two output pins is a differential signal swinging symmetrically around a common-mode voltage where that is the average voltage for the two outputs.
• Single-ended to differential—always use the outputs differentially in an FDA; however, the source signal can be either a single-ended source or differential, with a variety of implementation details for either. When the FDA operation is single-ended to differential, only one of the two input resistors receives the source signal with the other input resistor connected to a DC reference (often ground) or through a capacitor to ground.

To simplify, several features in the application of the LMH5485-SP are not explicitly stated, but are necessary for correct operation. These requirements include:

• Although not always stated, make sure to tie the power disable pin to the positive supply when only 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 termination 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 LMH5485-SP, and on to an ADC input do not require doubly-terminated lines or filter designs; the exception is if the source requires a defined termination impedance for correct operation (for example, a SAW filter source).
• External element values are normally assumed to be accurate and matched. In an FDA, match the feedback resistor values and also match the (DC and AC) impedance from the summing junctions to the source on one side and the reference or ground on the other side. Unbalancing these values introduces nonidealities in the signal path. For the signal path, imbalanced resistor ratios on the two sides create a common-mode to differential conversion. Also, mismatched Rf values and feedback ratios create some added differential output error terms from any common-mode DC, ac signal, or noise terms. Snapping to standard 1% resistor values is a typical approach and generally leads to some nominal feedback ratio mismatch. Mismatched resistors or ratios do not in themselves degrade harmonic distortion. If there is meaningful CM noise or distortion coming in, those errors are converted to a differential error through element or ratio mismatch.