OPA3693 Datasheet, PDF (22/29 Pages) Burr-Brown (TI) – Triple, Ultra-Wideband, Fixed-Gain, VIDEO BUFFER with Disable

OPA3693
SBOS353 – DECEMBER 2006
OPA3693 can be very susceptible to decreased
stability and may give closed-loop response peaking
when a capacitive load is placed directly on the
output pin. When the amplifier 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 simplest 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 resistor
does not eliminate the pole from the loop response,
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 Characteristics show a Recommended
R_S vs Capacitive Load curve (Figure 15) to help the
designer pick a value to give < 0.5dB peaking to the
load. The resulting frequency response curves show
a 0.5dB peaked response for several selected
capacitive loads and recommended R_S
combinations. Parasitic capacitive loads greater than
2pF can begin to degrade the performance of the
OPA3693. Long PCB traces, unmatched cables, and
connections to other amplifier inputs can easily
exceed this value. Always consider this effect
carefully, and add the recommended series resistor
as close as possible to the OPA3693 output pin (see
the Board Layout Guidelines section).
The criterion for setting this R_S resistor is a maximum
bandwidth, flat frequency response at the load
(< 0.5dB peaking). For the OPA3693 operating at a
gain of +2V/V, the frequency response at the output
pin is very flat to begin with, allowing relatively small
values of R_S to be used for low capacitive loads.
DISTORTION PERFORMANCE
The OPA3693 provides good distortion performance
into a 100Ω load on ±5V supplies. Relative to
alternative solutions, the OPA3693 holds much lower
distortion at higher frequencies (> 20MHz) than
alternative solutions. Generally, until the fundamental
signal reaches very high-frequency or power levels,
the 2nd-harmonic dominates 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 42), this value
is the sum of R_F + R_G, while in the inverting
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configuration it is just R_F (see Figure 44). Also,
providing an additional supply decoupling capacitor
(0.01µF) between the supply pins (for bipolar
operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
The OPA3693 has an extremely low 3rd-order
harmonic distortion. This feature also produces a
high two-tone, 3rd-order intermodulation intercept.
Two graphs for this intercept are given in the in the
Typical Characteristics; one for ±5V and one for +5V.
The lower curve shown in each graph is defined at
the 50Ω load when driven through a 50Ω matching
resistor, to allow direct comparisons to RF MMIC
devices. The higher curve in each graph shows the
intercept if the output is taken directly at the output
pin with a 500Ω load, to allow prediction of the
3rd-order spurious level when driving a lighter load,
such as an ADC input. The output matching resistor
attenuates the voltage swing from the output pin to
the load by 6dB. If the OPA3693 drives directly into
the input of a high-impedance device, such as an
ADC, this 6dB attenuation is not taken and the
intercept increases, as shown in the 500Ω load
typical characteristic.
The intercept is used to predict the intermodulation
spurious levels for two closely-spaced frequencies. If
the two test frequencies (f1 and f2) are specified in
terms of average and delta frequency, f_O = (f1 + f2)/2
and ∆f = |f2 – f1|/2, then the two, 3rd-order, close-in
spurious tones appear at f_O ±3 × ∆f. The difference
between two equal test tone power levels and these
intermodulation spurious power levels is given by
∆dBc = 2 × (IM3 – P_O), where IM3 is the intercept
taken from the Typical Characteristics and P_O is the
power level in dBm at the 50Ω load for one of the
two closely-spaced test frequencies. For instance, at
50MHz, the OPA3693 at a gain of +2 has an
intercept of 47dBm at a matched 50Ω load. If the full
envelope of the two frequencies needs to be 2V_PP at
this load, this requires each tone to be 4dBm (1V_PP).
The 3rd-order intermodulation spurious tones will
then be 2 × (47 – 4) = 83dBc below the test tone
power level (–79dBm). If this same 2V_PP two-tone
envelope were delivered directly into a lighter 500Ω
load, the intercept would increase to the 48dBm
shown in the Typical Characteristics. With the same
output signal and gain conditions, but now driving
directly into a light load with no matching loss, the
3rd-order spurious tones will then be at least 2 × (48
– 4) = 92dBc below the 4dBm test tone power levels
centered on 50MHz (–88dBm). We are still using a
4dBm for the 1V_PP output swing into this 500Ω load.
While not strictly correct from a power standpoint,
this does give the correct prediction for spurious
level. The class AB output stage for the OPA3693 is
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