THS3122 Datasheet, PDF (14/43 Pages) Texas Instruments – LOW-NOISE, HIGH-SPEED, 450 mA CURRENT FEEDBACK AMPLIFIERS

THS3122, THS3125
SLOS382E – SEPTEMBER 2001 – REVISED MAY 2015
8 Detailed Description
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8.1 Overview
The THS3122 and THS3125 family of dual-channels, bipolar-input, high-speed current feedback amplifiers offers
a low-noise of 2.2 nV/√Hz with a high output current drive of 450 mA. This performance is ideal for any
application that requires low distortion over a wide range of frequencies with heavy loads.
8.2 Feature Description
8.2.1 Maximum Slew Rate For Repetitive Signals
The THS3125 and THS3122 are recommended for high slew rate pulsed applications where the internal nodes
of the amplifier have time to stabilize between pulses. It is recommended to have at least 20-ns delay between
pulses.
The THS3125 and THS3122 are not recommended for applications with repetitive signals (sine, square,
sawtooth, or other) that exceed 900 V/µs. Using the part in these applications results in excessive current draw
from the power supply and possible device damage.
For applications with high slew rate, repetitive signals, the THS3091 and THS3095 (single versions), or THS3092
and THS3096 (dual versions) are recommended.
8.2.2 Saving Power with Shutdown Functionality and Setting Threshold Levels with the Reference Pin
The THS3125 features a shutdown pin (SHUTDOWN) that lowers the quiescent current from 8.4 mA/amp down
to 370 µA/amp, ideal for reducing system power.
The shutdown pin of the amplifier defaults to the REF pin voltage in the absence of an applied voltage, putting
the amplifier in the normal on mode of operation. To turn off the amplifier in an effort to conserve power, the
shutdown pin can be driven towards the positive rail. The threshold voltages for power-on and power-down (or
shutdown) are relative to the supply rails and are given in the Electrical Characteristics: Shutdown
Characteristics (THS3125 Only) table. Below the Enable threshold voltage, the device is on. Above the Disable
threshold voltage, the device is off. Behavior between these threshold voltages is not specified.
Note that this shutdown functionality is self-defining: the amplifier consumes less power in shutdown mode. The
shutdown mode is not intended to provide a high-impedance output. In other words, the shutdown functionality is
not intended to allow use as a 3-state bus driver. When in shutdown 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
shutdown mode. Most notably is the fact that the amplifier actually turns on if there is a ±0.7 V or greater
difference between the two input nodes (IN+ and IN–) of the amplifier. If this difference exceeds ±0.7 V, the
output of the amplifier creates an output voltage equal to approximately [(IN+ – IN–) – 0.7V] × Gain. Also, if a
voltage is applied to the output while in shutdown mode, the IN– 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 behavior described here.
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 microseconds because
the amplifier moves in and out of the linear mode of operation in these transitions.
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