THS4541_15 Datasheet, PDF (33/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

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9 Detailed Description
THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
9.1 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-V_PP, 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 (see Figure 63). 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 they would for any disabled FDA device.
9.1.1 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 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.
• Minimize the distance (< 0.1") from the power-supply pins to high-frequency, 0.1-μF decoupling capacitors. At
the device pins, the ground and power plane layout should not be in close proximity to the signal I/O pins.
Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling
capacitors. The power-supply connections (on pins 4 and 7) should always be decoupled with these
capacitors. An optional supply decoupling capacitor across the two power supplies (for bipolar operation)
improves 2nd-harmonic distortion performance. Larger (2.2μF to 6.8μF) decoupling capacitors, effective at
lower frequency, should also be used on the main supply pins. These can be placed somewhat farther from
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