THS4121IDGN Datasheet, PDF (15/30 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

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THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
PRINCIPLES OF OPERATION (continued)
Fully differential amplifiers may be viewed as two inverting amplifiers. In this case, the equation of an inverting
amplifier holds true for gain calculations. One advantage of fully differential amplifiers is that they offer twice as
much dynamic range compared to single-ended amplifiers. For example, a 1-V_PP ADC can only support an input
signal of 1 V_PP. If the output of the amplifier is 2 V_PP, then it is not practical to feed a 2-V_PP signal into the
targeted ADC. Using a fully differential amplifier enables the user to break down the output into two 1-V_PP
signals with opposite signs and feed them into the differential input nodes of the ADC. In practice, the designer
has been able to feed a 2-V peak-to-peak signal into a 1-V differential ADC with the help of a fully differential
amplifier. The final result indicates twice as much dynamic range. Figure 27 illustrates the increase in dynamic
range. The gain factor should be considered in this scenario. The THS412x fully differential amplifier offers an
improved CMRR and PSRR due to its symmetrical input and output. Furthermore, second harmonic distortion is
improved. Second harmonics tend to cancel because of the symmetrical output.
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
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 (2,54
mm) 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
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