Numerous common terms that are unique to this type of device exist. This section identifies and explains these terms.

• Fully differential amplifier (FDA). This term is restricted to devices offering what appears 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 (V_OCM) 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, which is the average voltage for the two outputs.
• Single-ended to differential. The output must always be used differentially in an FDA; however, the source signal can be either a single-ended or a differential source with a variety of implementation details for either source. For an FDA operating in single-ended to differential, only one of the two input signals is applied to one of the input resistors.
• The common-mode control has limited bandwidth from the input VOCM pin to the common-mode output voltage. The internal loop bandwidth beyond the input V_OCM buffer is a much wider bandwidth than the reported V_OCM bandwidth, but is not directly discernable. A very wide bandwidth in the internal V_OCM loop is required to perform an effective and low-distortion single-ended to differential conversion.

Several features in the application of the THS4551 are not explicitly stated, but are necessary for correct operation. These features are:

• Good power-supply decoupling is required. Often a larger capacitor (2.2 µF, typical) is used along with a high-frequency, 0.1-µF supply decoupling capacitor at the device supply pins (share this capacitor with the four supply pins in the RGT package). For single-supply operation, only the positive supply has these capacitors. Where a split supply is used, connect these capacitors to ground on both sides with the larger capacitor placed some distance from the package and shared among multiple channels of the THS4551, if used. A separate 0.1-µF capacitor must be provided to each device at the device power pins. With cascaded or multiple parallel channels, including ferrite beads from the larger capacitor to the local high-frequency decoupling capacitor is often useful.
• Although often not stated, the power disable pin ( PD) is tied 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 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 THS4551, 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- 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 TH4551 is less than 5.5 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. The disable pin ( PD) is referenced to the negative rail. Using a negative supply requires that PD be pulled down to within 0.55 V of the negative supply to disable the amplifier.
• 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 R_F 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 coming in 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 (1% steps).