Datasheet

Driving Capacitive Loads Distortion Performance
OPA659
SBOS342B – DECEMBER 2008 – REVISED AUGUST 2009 ............................................................................................................................................ www.ti.com
One of the most demanding, and yet very common, The OPA659 is capable of delivering a low distortion
load conditions for an op amp is capacitive loading. signal at high frequencies over a wide range of gains.
The OPA659 is very robust, but care should be taken The distortion plots in the Typical Characteristics
with light loading scenarios so that output show the typical distortion under a wide variety of
capacitance does not decrease stability and increase conditions. Generally, until the fundamental signal
closed-loop frequency response peaking when a reaches very high frequencies or powers, the second
capacitive load is placed directly on the output pin. harmonic dominates the distortion with a negligible
When the amplifier open-loop output resistance is third harmonic component. Focusing then on the
considered, this capacitive load introduces an second harmonic, increasing the load impedance
additional pole in the signal path that can decrease improves distortion directly. Remember that the total
the phase margin. Several external solutions to this load includes the feedback network: in the
problem have been suggested. When the primary noninverting configuration, this network is the sum of
considerations are frequency response flatness, R
F
+ R
G
, while in the inverting configuration the
pulse response fidelity, and/or distortion, the simplest network is only R
F
(see Figure 35 ). Increasing the
and most effective solution is to isolate the capacitive output voltage swing directly increases harmonic
load from the feedback loop by inserting a series distortion. A 6dB increase in output swing generally
isolation resistor, R
ISO
, between the amplifier output increases the second harmonic by 12dB and the third
and the capacitive load. In effect, this resistor isolates harmonic by 18dB. Increasing the signal gain also
the phase shift from the loop gain of the amplifier, increases the second-harmonic distortion. Again, a
thus increasing the phase margin and improving 6dB increase in gain increases the second and third
stability. The Typical Characteristics show the harmonics by about 6dB, even with a constant output
recommended R
ISO
versus capacitive load and the power and frequency. Finally, the distortion increases
resulting frequency response with a 1k Ω load (see as the fundamental frequency increases because of
Figure 24 ). Note that larger R
ISO
values are required the rolloff in the loop gain with frequency. Conversely,
for lower capacitive loading. In this case, a design the distortion improves going to lower frequencies,
target of a maximally-flat frequency response was down to the dominant open-loop pole at
used. Lower values of R
ISO
may be used if some approximately 300kHz.
peaking can be tolerated. Also, operating at higher
Note that power-supply decoupling is critical for
gains (instead of the +1 gain used in the Typical
harmonic distortion performance. In particular, for
Characteristics ) requires lower values of R
ISO
for a
optimal second-harmonic performance, the
minimally-peaked frequency response. Parasitic
power-supply high-frequency 0.1 µ F decoupling
capacitive loads greater than 2pF can begin to
capacitors to the positive and negative supply pins
degrade the performance of the OPA659. Moreover,
should be brought to a single point ground located
long PCB traces, unmatched cables, and connections
away from the input pins.
to multiple devices can easily cause this value to be
exceeded. Always consider this effect carefully, and
The OPA659 has an extremely low third-order
add the recommended series resistor as close as
harmonic distortion. This characteristic also shows up
possible to the OPA659 output pin (see the Board
in the two-tone, third-order intermodulation spurious
Layout section).
(IMD3) response curves (see Figure 19 ). The
third-order spurious levels are extremely low (less
With heavier loads (for example, the 100 Ω load
than – 100dBc) at low output power levels and
presented in the test circuits and used for testing
frequencies below 10MHz. The output stage
typical characteristic performance), the OPA659 is
continues to hold these levels low even as the
very robust; R
ISO
can be as low as 10 Ω with
fundamental power reaches higher levels. As with
capacitive loads less than 5pF and continue to show
most op amps, the spurious intermodulation powers
a flat frequency response.
do not increase as predicted by a traditional intercept
space model. As the fundamental power level increases, the
dynamic range does not decrease significantly. For
space
two tones centered at 10MHz, with – 2dBm/tone into a
matched 50 Ω load (that is, 0.5V
PP
for each tone at
the load, which requires 2V
PP
for the overall two-tone
envelope at the output pin), the Typical
Characteristics show a 96dBc difference between the
test tones and the third-order intermodulation
spurious levels. This exceptional performance
improves further when operating at lower frequencies
and/or higher load impedances.
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