OPA4684M Datasheet, PDF (26/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
The Typical Characteristics show the recommended RS vs CLOAD and the resulting frequency response at the
load. The 1-kΩ resistor shown in parallel with the load capacitor is a measurement path and may be omitted.
Parasitic capacitive loads greater than 5 pF can begin to degrade the performance of the OPA4684. Long PC
board traces, unmatched cables, and connections to multiple devices can easily cause this value to be
exceeded. Always consider this effect carefully, and add the recommended series resistor as close as possible
to the OPA4684 output pin (see Board Layout Guidelines).
distortion performance
The OPA4684 provides very low distortion in a low-power part. The CFBPLUS architecture also gives two
significant areas of distortion improvement. First, in operating regions where the 2nd-harmonic distortion due
to output stage nonlinearities is very low (frequencies < 1MHz, low output swings into light loads) the
linearization at the inverting node provided by the CFBPLUS design gives 2nd-harmonic distortions that extend
into the −90 dBc region. Previous current-feedback amplifiers have been limited to approximately −85 dBc due
to the nonlinearities at the inverting input. The second area of distortion improvement comes in a distortion
performance that is largely gain independent. To the extent that the distortion at a particular output power is
output stage dependent, 3rd-harmonics particularly, and to a lesser extend 2nd-harmonic distortion, is constant
as the gain is increased. This is due to the constant loop gain versus signal gain provided by the CFBPLUS
design. As shown in the Typical Characteristic curves, while the 3rd-harmonic is constant with gain, the
2nd-harmonic degrades at higher gains. This is largely due to board parasitic issues. Slightly imbalanced load
return currents will couple into the gain resistor to cause a portion of the 2nd-harmonic distortion. At high gains,
this imbalance has more gain to the output giving reduced 2nd-harmonic distortion. Differential stages using
two of the channels together can reduce this 2nd-harmonic issue enormously getting back to an essentially gain
independent distortion.
Relative to alternative amplifiers with < 2-mA/ch supply current, the OPA4684 holds much lower distortion at
higher frequencies (> 5 MHz) and to higher gains. Generally, until the fundamental signal reaches very high
frequency or power levels, the 2nd-harmonic will dominate the distortion with a lower 3rd-harmonic component.
Focusing then on the 2nd-harmonic, increasing the load impedance improves distortion directly. Remember that
the total load includes the feedback network−in the noninverting configuration (see Figure 41) this is the sum
of RF + RG, while in the inverting configuration it is just RF. Also, providing an additional supply decoupling
capacitor (0.1 µF) between the supply pins (for bipolar operation) improves the 2nd-order distortion slightly (3dB
to 6dB).
In most op amps, increasing the output voltage swing increases harmonic distortion directly. A low-power part
like the OPA4684 includes quiescent boost circuits to provide the large-signal bandwidth in the Electrical
Characteristics. These act to increase the bias in a very linear fashion only when high slew rate or output power
are required. This also acts to actually reduce the distortion slightly at higher output power levels. The Typical
Characteristic curves show the 2nd-harmonic holding constant from 500 mVp-p to 5 Vp-p outputs while the
3rd-harmonics actually decrease with increasing output power.
The OPA4684 has an extremely low 3rd-order harmonic distortion, particularly for light loads and at lower
frequencies. This also gives low 2-tone, 3rd-order intermodulation distortion as shown in the Typical
Characteristic curves. Since the OPA4684 includes internal power boost circuits to retain good full-power
performance at high frequencies and outputs, it does not show a classical 2-tone, 3rd-order intermodulation
intercept characteristic. Instead, it holds relatively low and constant 3rd-order intermodulation spurious levels
over power. The Typical Characteristic curves show this spurious level as a dBc below the carrier at fixed center
frequencies swept over single-tone power at a matched 50-Ω load. These spurious levels drop significantly (>
12 dB) for lighter loads than the 100 Ω used in the 2-Tone, 3rd-Order Intermodulation Distortion curve. Converter
inputs for instance will see < −82dBc 3rd-order spurious to 10 MHz for full-scale inputs. For even lower 3rd-order
intermodulation distortion to much higher frequencies, consider the OPA2691 dual or OPA691 and OPA685
single-channel current-feedback amplifiers.
26
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265