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APPLICATION INFORMATION

For many years, high-performance analog design has required the generation of split power supply voltages, like

SNR +

Signal

Noise

SFDR +

Signal

Spur

GENERAL APPLICATION

HARMONIC DISTORTION

THS4304

SLOS436A – MARCH 2004 – REVISED JULY 2004

±15 V, ±8 V, and more recently ±5 V, in order to realize the full performance of the amplifiers available. Modern

trends in high-performance analog are moving towards single-supply operation at 5 V, 3 V, and lower. This

reduces power supply cost due to less voltages being generated and conserves energy in low power

applications. It can also take a toll on available dynamic range, a valuable commodity in analog design, if the

available voltage swing of the signal must also be reduced.

Two key figures of merit for dynamic range are signal-to-noise ratio (SNR) and spurious free dynamic range

(SFDR).

SNR is simply the signal level divided by the noise:

and SFDR is the signal level divided by the highest spur:

In an operational amplifier, reduced supply voltage typically results in reduced signal levels due to lower voltage

available to operate the transistors within the amplifier. When noise and distortion remain constant, the result is a

commensurate reduction in SNR and SFDR. To regain dynamic range, the process and the architecture used to

make the operational amplifier must have superior noise and distortion performance with lower power supply

overhead required for proper transistor operation.

The THS4304 BiCom3 operational amplifier is just such a device. It is able to provide 2-Vpp signal swing at its

output on a single 5-V supply with noise and distortion performance similar to the best 10-V operational

amplifiers on the market today

The THS4304 is a traditional voltage-feedback topology with wideband performance up to 3 GHz at unity gain.

Care must be taken to ensure that parasitic elements do not erode the phase margin.

Capacitance at the output and inverting input, and resistance and inductance in the feedback path, can cause

problems.

To reduce parasitic capacitance, the ground plane should be removed from under the part.

To reduce inductance in the feedback, the circuit traces should be kept as short and direct as possible. For best

performance in non-inverting unity gain (G=+1V/V), it is recommended to use a wide trace directly between the

output and inverting input.

For a gain of +2V/V, it is recommended to use a 249-

Ω feedback resistor. With good layout, this should keep the

frequency response peaking to around 2 dB. This resistance is high enough to not load the output excessively,

and the part is capable of driving 100-

Ω load with good performance. Higher-value resistors can be used, with

more peaking. For example, 499

Ω gives about 5 dB of peaking, and gives slightly better distortion performance

with 100-

Ω load. Lower value feedback resistors can also be used to reduce peaking, but degrades the distortion

performance with heavy loads.

Power supply bypass capacitors are required for proper operation. The most critical are 0.1-µF ceramic

capacitors; these should be placed as close to the part as possible. Larger bulk capacitors can be shared with

other components in the same area as the operational amplifier.

For best second harmonic (HD2), it is important to use a single-point ground between the power supply bypass

capacitors when using a split supply. It is also recommended to use a single ground or reference point for input

termination and gain-setting resistors (R8 and R11 in the non-inverting circuit). It is recommended to follow the

EVM layout closely in your application.

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