THS4304 Datasheet, PDF (15/26 Pages) Texas Instruments – Wideband Operational Amplifier

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THS4304
APPLICATION INFORMATION
SLOS436A – MARCH 2004 – REVISED JULY 2004
For many years, high-performance analog design has required the generation of split power supply voltages, like
±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:
Signal
SNR + _Noise
and SFDR is the signal level divided by the highest spur:
SFDR
+
Signal
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
GENERAL APPLICATION
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.
HARMONIC DISTORTION
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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