THS4551 Datasheet, PDF (26/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
9 Detailed Description
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9.1 Overview
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
V_CM error amplifier where the voltage is compared to either a default voltage divider across the supplies or an
externally set V_OCM 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 Functional Block Diagram).
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.
9.2 Functional Block Diagram
(RGT Package) FB+
VS+
3.3
0.6 pF
IN±
1.2 pF
IN+
±
High-AOL +
Differential I/O
Amplifier ±
+
0.6 pF
5.2 k
5.2 k
OUT+
OUT±
(RGT Package) FB±
3.3
VS±
PD
CMOS
Buffer
VS+
300 k
±
VCM
Error
Amplifier
+
300 k
VOCM
VS±
26
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