THS4561 Datasheet, PDF (18/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

THS4561
SBOS874 – AUGUST 2017
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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 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 THS4561 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 THS4561, 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 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- 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. The disable pin (PD) is referenced to the negative rail. Using a negative supply
requires that PD be pulled down to within TBD 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).
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