THS3120_16 Datasheet, PDF (18/35 Pages) Texas Instruments – LOW-NOISE, HIGH-OUTPUT DRIVE, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3120
THS3121
SLOS420E – SEPTEMBER 2003 – REVISED OCTOBER 2009........................................................................................................................................ www.ti.com
F_IN
R_G
124 Ω
V_S
R_F
499 Ω
27 pF
V_S
FB
_
+
-V_S
49.9 Ω
5.11 Ω
100 Ω LOAD
1 µF
Figure 55. Feedback Technique with Input Ferrite
Bead for Capacitive Load
Figure 56 is shown using two amplifiers in parallel to
double the output drive current to larger capacitive
loads. This technique is used when more output
current is needed to charge and discharge the load
faster as when driving large FET transistors.
Figure 57 shows a push-pull FET driver circuit typical
of ultrasound applications with isolation resistors to
isolate the gate capacitance from the amplifier.
V_S
V_S
+
5.11 Ω
_
-V_S
301 Ω
66.5 Ω
301 Ω
V_S
_
5.11 Ω
+
-V_S
-V_S
Figure 57. PowerFET Drive Circuit
499 Ω
124 Ω
24.9 Ω
V_S
_
+
-V_S
499 Ω
5.11 Ω
V_S
V_S
1 nF
124 Ω
_
5.11 Ω
+
24.9 Ω
-V_S
Figure 56. Parallel Amplifiers for Higher Output
Drive
SAVING POWER WITH POWER-DOWN
FUNCTIONALITY AND SETTING
THRESHOLD LEVELS WITH THE
REFERENCE PIN
The THS3120 features a power-down pin (PD) which
lowers the quiescent current from 7 mA down to
300 μA, ideal for reducing system power.
The power-down 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 power-down pin can be driven
towards the positive rail. The threshold voltages for
power-on and power-down are relative to the supply
rails and are given in the specification tables. Below
the Enable Threshold Voltage, the device is on.
Above the Disable Threshold Voltage, the device is
off. Behavior in between these threshold voltages is
not specified.
Note that this power-down functionality is just that;
the amplifier consumes less power in power-down
mode. The power-down mode is not intended to
provide a high-impedance output. In other words, the
power-down functionality is not intended to allow use
as a 3-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.
18
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