OPA3681 Datasheet, PDF (18/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common load
conditions for an op amp is capacitive loading. Often, the
capacitive load is the input of an A/D converter—including
additional external capacitance which may be recommended
to improve A/D linearity. A high speed, high open-loop gain
amplifier like the OPA3681 can be very susceptible to
decreased stability and closed-loop response peaking when
a capacitive load is placed directly on the output pin. When
the amplifier’s open-loop output resistance is considered,
this capacitive load introduces an additional pole in the
signal path that can decrease the phase margin. Several
external solutions to this problem have been suggested.
When the primary considerations are frequency response
flatness, pulse response fidelity and/or distortion, the sim-
plest and most effective solution is to isolate the capacitive
load from the feedback loop by inserting a series isolation
resistor between the amplifier output and the capacitive
load. This does not eliminate the pole from the loop re-
sponse, but rather shifts it and adds a zero at a higher
frequency. The additional zero acts to cancel the phase lag
from the capacitive load pole, thus increasing the phase
margin and improving stability.
The Typical Performance Curves show the recommended
R_S vs Capacitive Load and the resulting frequency response
at the load. Parasitic capacitive loads greater than 2pF can
begin to degrade the performance of the OPA3681. 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 OPA3681 output
pin (see Board Layout Guidelines).
DISTORTION PERFORMANCE
The OPA3681 provides good distortion performance into a
100Ω load on ±5V supplies. Relative to alternative solu-
tions, it provides exceptional performance into lighter loads
and/or operating on a single +5V supply. 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 non-inverting configuration
(Figure 1), 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 (0.1µF) between the supply
pins (for bipolar operation) improves the 2nd-order distor-
tion slightly (3dB to 6dB).
In most op amps, increasing the output voltage swing in-
creases harmonic distortion directly. The Typical Performance
Curves show the 2nd harmonic increasing at a little less than
the expected 2x rate while the 3rd harmonic increases at a little
less than the expected 3x rate. Where the test power doubles,
the difference between it and the 2nd harmonic decreases less
than the expected 6dB while the difference between it and the
3rd decreases by less than the expected 12dB. This also shows
up in the 2-tone, 3rd-order intermodulation spurious (IM3)
response curves. The 3rd- order spurious levels are extremely
low at low output power levels. The output stage continues to
hold them low even as the fundamental power reaches very
high levels. As the Typical Performance Curves show, the
spurious intermodulation powers do not increase as predicted
by a traditional intercept model. As the fundamental power
level increases, the dynamic range does not decrease signifi-
cantly. For 2 tones centered at 20MHz, with 10dBm/tone into
a matched 50Ω load (i.e., 2Vp-p for each tone at the load,
which requires 8Vp-p for the overall 2-tone envelope at the
output pin), the Typical Performance Curves show 62dBc
difference between the test tone power and the 3rd-order
intermodulation spurious levels. This exceptional perfor-
mance improves further when operating at lower frequencies.
NOISE PERFORMANCE
Wideband current feedback op amps generally have a higher
output noise than comparable voltage-feedback op amps.
The OPA3681 offers an excellent balance between voltage
and current noise terms to achieve low output noise. The
inverting current noise (15pA/√Hz) is significantly lower
than earlier solutions while the input voltage noise
(2.2nV/√Hz) is lower than most unity gain stable, wideband,
voltage feedback op amps. This low input voltage noise was
achieved at the price of higher non-inverting input current
noise (12pA/√Hz). As long as the AC source impedance
looking out of the non-inverting node is less than 100Ω, this
current noise will not contribute significantly to the total
output noise. The op amp input voltage noise and the two
input current noise terms combine to give low output noise
under a wide variety of operating conditions. Figure 12
shows the op amp noise analysis model with all the noise
terms included. In this model, all noise terms are taken to be
noise voltage or current density terms in either nV/√Hz or
pA/√Hz.
E_NI
1/3
R_S
I_BN
OPA3681
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 12. Op Amp Noise Analysis Model.
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OPA3681
18