SBOS778D April 2016 – April 2021 THS4551
In addition to the core differential I/O voltage feedback gain block, there are two 5.2-kΩ resistors internally across the outputs to sense the average voltage at the outputs. These resistors feed the average voltage back into a VCM error amplifier where the voltage is compared to either a default voltage divider across the supplies or an externally set VOCM target voltage. When the amplifier is disabled, the default midsupply bias string is disabled to save power.
To achieve the very-low noise at the low power provided by the THS4551, the input stage transistors are relatively large, thus resulting in a higher differential input capacitance (1.2 pF in the Section 9.2). As a default compensation for the 1.2-pF differential input capacitance and the 1-kΩ feedback resistors used in characterization, internal 0.6-pF capacitors are placed between the two output and input pins. Adjust any desired external feedback capacitor value to account for these 0.6-pF internal elements. When using the 16-pin WQFN package and the internal feedback traces to the input side of the package, include the nominal trace impedance of 3.3 Ω in the design. These elements are not included in the TINA-TI™ model and must be added externally to a design intending to use the RGT package.