THS3491 Datasheet, PDF (10/28 Pages) Texas Instruments – 900-MHz, High-Power Output, Current Feedback Amplifier

THS3491
SBOS875 – AUGUST 2017
8 Detailed Description
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8.1 Overview
The THS3491 is a high-voltage, low-distortion, high-speed, current-feedback amplifier designed to operate over a
wide supply range of ±7 V to ±16 V for applications requiring large, linear output swings such as arbitrary
waveform generators.
The THS3491 features a power-down pin that puts the amplifier in low power standby mode, and lowers the
quiescent current from 16.5 mA to 750 µA.
8.2 Functional Block Diagram
VSIG
VREF
VIN+ ₊
.(s)
THS3491
VIN±
ZIN_±
±
Ierr
(1 + RF/RG)VSIG
VO
VREF
ZOL(s)xIerr
RF
AV = VO/VIN+ = 1 + (RF/RG)
RG
VREF
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8.3 Feature Description
8.3.1 Power-Down (PD) Pin
The THS3491 features a power-down (PD) pin that lowers the quiescent current from 16.8 mA down to 800 µA,
ideal for reducing system power.
The power-down pin of the amplifier defaults to 2 V below the positive supply voltage in the absence of an
externally applied voltage, putting the amplifier in the power-on mode of operation. To turn off the amplifier in an
effort to conserve power, the power-down pin can be driven towards the negative rail. The threshold voltages for
power on and power down are relative to the REF pin and are given in the Electrical Characteristics: V_S = ±15 V
and Electrical Characteristics: V_S = ±7.5 V tables. Above the enable threshold voltage, the device is on. Below
the disable threshold voltage, the device is off. Behavior in between these threshold voltages is not specified.
This power-down functionality helps the amplifier consume less power in power-down mode. Power-down mode
is not intended to provide a high-impedance output. In other words, the power-down functionality is not intended
for use as a tri-state bus driver. When in power-down mode, the impedance looking back into the output of the
amplifier is dominated by the feedback and gain-setting resistors, but the output impedance of the device itself
varies depending on the voltage applied to the outputs.
As with most current-feedback amplifiers, the internal architecture places some limitations on the system when in
power-down mode. Most notably is the fact that the amplifier actually turns on if there is a ±1 V or greater
difference between the two input nodes (VIN+ and VIN–) of the amplifier. If this difference exceeds ±1 V, the
output of the amplifier creates an output voltage equal to approximately [(VIN+ – VIN–) –0.7 V] × gain. This also
implies that if a voltage is applied to the output while in power-down mode, the VIN– node voltage is equal to
V_O(applied) × R_G / (R_F + R_G). For low-gain configurations and a large applied voltage at the output, the amplifier
may actually turn on because of the aforementioned behavior.
The time delays associated with turning the device on and off are specified as the time it takes for the amplifier to
reach either 10% or 90% of the final output voltage. The time delays are in the order of nanoseconds during
power on, and microseconds during power off because the amplifier moves out of the linear mode of operation
for power-off condition.
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