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CIRCUIT LAYOUT CONSIDERATIONS

POWER-DOWN MODE

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

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.

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.

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-k

Ω 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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