THS4304-SP Datasheet, PDF (12/22 Pages) Texas Instruments – RAD-TOLERANT CLASS V, WIDEBAND OPERATIONAL AMPLIFIER

THS4304-SP
SGDS038A – SEPTEMBER 2007 – REVISED OCTOBER 2007
APPLICATION INFORMATION
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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, to realize the full performance of the amplifiers available. Modern trends in
high-performance analog are moving toward single-supply operation at 5 V, 3 V, and lower. This reduces
power-supply cost due to less voltage being generated and conserves energy in low-power applications. It also
can take a toll on available dynamic range, a valuable commodity in analog design, if the available voltage swing
of the signal also must 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:
SNR
+
Signal
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 1 GHz at a gain of
2 V/V. 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 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 6 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. Lower-value feedback resistors also can be used to reduce peaking, but degrade 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 also is 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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