THS4541IRUNR.B datasheet(33/66 Pages) TI2

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8 Detailed Description

8.1 Overview

The THS4541 is a voltage-feedback (VFA) based, fully-differential amplifier (FDA) offering greater than 500-

MHz, small-signal bandwidth at a gain of 2 V/V with trimmed supply current and input offset voltage. The

core differential amplifier is a slightly decompensated voltage-feedback design with a high slew-rate, precision

input stage. This design gives the 500-MHz gain of 2-V/V small-signal bandwidth shown in the characterization

curves, with a 1500-V/µs slew rate, yielding approximately a 340-MHz, 2-VPP, large-signal bandwidth in the

same circuit configuration.

The outputs offer near rail-to-rail output swing (0.2-V headroom to either supply), while the device inputs are

negative rail inputs with approximately 1.2 V of headroom required to the positive supply. This negative rail input

directly supports a bipolar input around ground in a dc-coupled, single-supply design( Figure 7-3 shows). Similar

to all FDA devices, the output average voltage (common-mode) is controlled by a separate common-mode

loop. The target for this output average is set by the Vocm input pin that can be either floated to default

near midsupply or driven to a desired output common-mode voltage. The Vocm range extends from a very

low 0.91 V above the negative supply to 1.1 V below the positive supply, supporting a wide range of modern

analog-to-digital converter (ADC) input common-mode requirements using a single 2.7-V to 5.4-V supply range

for the THS4541.

A power-down pin (PD) is included. Pull the PD pin voltage to the negative supply to turn the device off, putting

the THS4541 into a very-low quiescent current state. For normal operation, the PD pin must be asserted high.

When the device is disabled, remember that the signal path is still present through the passive external resistors.

Input signals applied to a disabled THS4541 still appear at the outputs at some level through this passive

resistor path, as is the case for any disabled FDA device.

8.1.1 Terminology and Application Assumptions

Like all widely-used devices, numerous common terms have developed that are unique to this type of device.

These terms include:

• Fully differential amplifier (FDA)—In this document, this term is restricted to devices offering what appears

similar to a differential inverting op amp design element that requires an input resistor (not high-impedance

input) and includes a second internal control-loop setting the output average voltage (Vocm) to a default or

set point. This second loop interacts with the differential loop in some configurations.

• The desired output signal at the two output pins is a differential signal swinging symmetrically around a

common-mode voltage where that is the average voltage for the two outputs.

• Single-ended to differential—always use the outputs differentially in an FDA; however, the source signal can

be either a single-ended source or differential, with a variety of implementation details for either. When the

FDA operation is single-ended to differential, only one of the two input resistors receives the source signal

with the other input resistor connected to a dc reference (often ground) or through a capacitor to ground.

To simplify, several features in the application of the THS4541 are not explicitly stated, but are necessary for

correct operation. These requirements include:

• Good power-supply decoupling is required. Minimize the distance (< 0.1") from the power-supply pins to

high-frequency, 0.1-μF decoupling capacitors. Often a larger capacitor (2.2 µF is typical) is used along with a

high-frequency, 0.1-µF supply decoupling capacitor at the device supply pins (share this capacitor for the four

supply pins in the RGT package). For single-supply operation, only the positive supply has these capacitors.

When a split supply is used, use these capacitors for each supply to ground. If necessary, place the larger

capacitors somewhat farther from the device and share these capacitors among several devices in the same

area of the printed circuit board (PCB). For each THS4541, attach a separate 0.1-µF capacitor to a nearby

ground plane. With cascaded or multiple parallel channels, including ferrite beads from the larger capacitor is

often useful to the local high-frequency decoupling capacitor.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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