THS4120 Datasheet, PDF (16/23 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
PRINCIPLES OF OPERATION (continued)
V_IN-
V_IN+
V_DD
_
+
_
+
a
+1
V_O+
0
V_O-
+1
V_OD= 1-0 = 1
V_OCM
V_SS
0
b
V_OD = 0-1 = -1
Figure 27. Fully Differential Amplifier With Two 1-V_PP Signals
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CIRCUIT LAYOUT CONSIDERATIONS
To achieve the levels of high-frequency performance of the THS412x, follow proper printed-circuit board high
frequency design techniques. A general set of guidelines is given below. In addition, a THS412x evaluation board
is available to use as a guide for layout or for evaluating the device performance.
• Ground planes - It is highly recommended that a ground plane be used on the board to provide all components with a
low inductive ground connection. However, in the areas of the amplifier inputs and output, the ground plane can be
removed to minimize the stray capacitance.
• Proper power supply decoupling - Use a 6.8-µF tantalum capacitor in parallel with a 0.1-µF ceramic capacitor on each
supply terminal. It may be possible to share the tantalum among several amplifiers depending on the application, but a
0.1-µF ceramic capacitor should always be used on the supply terminal of every amplifier. In addition, the 0.1-µF
capacitor should be placed as close as possible to the supply terminal. As this distance increases, the inductance in the
connecting trace makes the capacitor less effective. The designer should strive for distances of less than 0.1 inch
between the device power terminals and the ceramic capacitors.
• Sockets - Sockets are not recommended for high-speed operational amplifiers. The additional lead inductance in the
socket pins often lead to stability problems. Surface-mount packages soldered directly to the printed-circuit board is the
best implementation.
• Short trace runs/compact part placements - Optimum high-frequency performance is achieved when stray series
inductance has been minimized. To realize this, the circuit layout should be made as compact as possible, thereby
minimizing the length of all trace runs. Particular attention should be paid to the inverting input of the amplifier. Its length
should be kept as short as possible. This helps to minimize stray capacitance at the input of the amplifier.
• Surface-mount passive components - Using surface-mount passive components is recommended for high-frequency
amplifier circuits for several reasons. First, because of the extremely low lead inductance of surface-mount components,
the problem with stray series inductance is greatly reduced. Second, the small size of surface-mount components
naturally leads to a more compact layout thereby minimizing both stray inductance and capacitance. If leaded
components are used, it is recommended that the lead lengths be kept as short as possible.
POWER-DOWN MODE
The THS4120 features a power-down pin (PD) which lowers the quiescent current from 11 mA down to 120 µA,
ideal for reducing system power. The power-down pin of the amplifier must be pulled high via a 10-kΩ pullup
resistor between the PD pin and the positive supply (see Figure 28) in the absence of an applied voltage, putting
the amplifier in the power-on mode of operation. To turn off (disable) the amplifier in an effort to conserve power,
the power-down pin can be driven towards the negative rail or ground. The threshold voltages for power-on and
power-down are relative to the supply rails and given in the specification 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.
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. 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.
The time delays associated with turning the device on and off are specified as the time it takes for the amplifier to
reach 50% of the nominal quiescent current. The enable time delay is in the order of microseconds due to the
amplifier moving in and out of the linear mode of operation.
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