Datasheet

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OPA694
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OPA694
www.ti.com SBOS319G –SEPTEMBER 2004–REVISED JANUARY 2010
DRIVING CAPACITIVE LOADS a little less than the expected 2x rate, while the
3rd-harmonic increases at a little less than the
One of the most demanding and yet very common
expected 3x rate. Where the test power doubles, the
load conditions for an op amp is capacitive loading.
2nd-harmonic increases by less than the expected
Often, the capacitive load is the input of an
6dB, while the 3rd-harmonic increases by less than
ADC—including additional external capacitance that
the expected 12dB. This also shows up in the
may be recommended to improve ADC linearity. A
two-tone, third-order intermodulation spurious (IM
3
)
high-speed, high open-loop gain amplifier like the
response curves. The 3rd-order spurious levels are
OPA694 can be very susceptible to decreased
extremely low at low output power levels. The output
stability and closed-loop response peaking when a
stage continues to hold them low even as the
capacitive load is placed directly on the output pin.
fundamental power reaches very high levels. As the
When the amplifier open−loop output resistance is
Typical Characteristics show, the spurious
considered, this capacitive load introduces an
intermodulation powers do not increase as predicted
additional pole in the signal path that can decrease
by a traditional intercept model. As the fundamental
the phase margin. Several external solutions to this
power level increases, the dynamic range does not
problem have been suggested. When the primary
decrease significantly.
considerations are frequency response flatness,
pulse response fidelity, and/or distortion, the simplest
NOISE PERFORMANCE
and most effective solution is to isolate the capacitive
load from the feedback loop by inserting a series
Wideband, current-feedback op amps generally have
isolation resistor between the amplifier output and the
a higher output noise than comparable
capacitive load. This does not eliminate the pole from
voltage-feedback op amps. The OPA694 offers an
the loop response, but rather shifts it and adds a zero
excellent balance between voltage and current noise
at a higher frequency. The additional zero acts to
terms to achieve low output noise. The inverting
cancel the phase lag from the capacitive load pole,
current noise (24pA/√Hz) is significantly lower than
thus increasing the phase margin and improving
earlier solutions, while the input voltage noise
stability.
(2.1nV/√Hz) is lower than most unity-gain stable,
wideband, voltage-feedback op amps. This low input
The Typical Characteristics show the recommended
voltage noise was achieved at the price of higher
R
S
vs Capacitive Load (Figure 15) and the resulting
noninverting input current noise (22pA/√Hz). As long
frequency response at the load. Parasitic capacitive
as the AC source impedance looking out of the
loads greater than 2pF can begin to degrade the
noninverting node is less than 100Ω, this current
performance of the OPA694. Long PCB traces,
noise will not contribute significantly to the total
unmatched cables, and connections to multiple
output noise. The op amp input voltage noise and the
devices can easily cause this value to be exceeded.
two input current noise terms combine to give low
Always consider this effect carefully, and add the
output noise under a wide variety of operating
recommended series resistor as close as possible to
conditions. Figure 40 shows the op amp noise
the OPA694 output pin (see the Board Layout
analysis model with all the noise terms included. In
Guidelines section).
this model, all noise terms are taken to be noise
voltage or current density terms in either nV/√Hz or
DISTORTION PERFORMANCE
pA/√Hz.
The OPA694 provides good distortion performance
into a 100Ω load on ±5V supplies. Generally, until the
fundamental signal reaches very high frequency or
power levels, the 2nd-harmonic will dominate the
distortion with a negligible 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 31), this is the sum of R
F
+ R
G
, while in the
inverting configuration it is just R
F
. Also, providing an
additional supply decoupling capacitor (0.1mF)
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
Figure 40. Op Amp Noise Analysis Model
increases harmonic distortion directly. The Typical
Characteristics show the 2nd-harmonic increasing at
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