THS4521 datasheet(35/66 Pages) TI2

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8.2 Functional Block Diagram

Vs+

IN±

High-Aol

Differential I/O

Amplifier

IN+

FB+

100 k

2.5 k

OUT+

OUT±

FB±

Vcm

Error

Amplifier

Vocm

Vs+

100 k

CMOS

Buffer

Vs±

(RGT Package)

8.3 Feature Description

8.3.1 Differential I/O

The THS4541 combines a core differential I/O, high-gain block with an output common-mode sense that is

compared to a reference voltage and then fed back into the main amplifier block to control the average output

to that reference. The differential I/O block is a classic, high open-loop gain stage with a dominant pole at

approximately 900 Hz. This voltage feedback structure projects a single-pole, unity-gain Aol at 850 MHz (gain

bandwidth product). The high-speed differential outputs include an internal averaging resistor network to sense

the output common-mode voltage. This voltage is compared by a separate Vcm error amplifier to the voltage

on the Vocm pin. If floated, this reference is at half the total supply voltage across the device using two

100-kΩ resistors. This Vcm error amplifier transmits a correction signal into the main amplifier to force the

output average voltage to meet the target voltage on the Vocm pin. The bandwidth of this error amplifier is

approximately the same bandwidth as the main differential I/O amplifier.

The differential outputs are collector outputs to obtain the rail-to-rail output swing. These outputs are relatively

high-impedance, open-loop sources; however, closing the loop provides a very low output impedance for load

driving. No output current limit or thermal shutdown features are provided in this lower-power device. The

differential inputs are PNP inputs to provide a negative-rail input range.

To operate the THS4541 for the RGT package, connect external resistors from the FB– pin to the IN+ pins, and

the FB+ pin to the IN– pins. For the RUN package, connect the OUT– pin to the IN+ pin through an Rf, and the

OUT+ pin to the IN– pin through the same value of Rf. Bring in the inputs through additional resistors to the IN+

and IN– pins. The differential I/O op amp operates similarly to an inverting op amp structure where the source

must drive the input resistor and the gain is the ratio of the feedback to the input resistor.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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