OPA2695 Datasheet, PDF (25/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
www.ti.com..................................................................................................................................................................................................... SBOS354 – APRIL 2008
The two noninverting inputs provide an easy
common-mode control input. This is particularly easy
if the source is ac-coupled through either blocking
caps or a transformer. In either case, the
common-mode input voltages on the two noninverting
inputs again have a gain of 1 to the output pins,
giving particularly easy common-mode control for
single-supply operation. The OPA2695 used in this
configuration does constrain the feedback to the
500Ω region for best frequency response. With R_F
fixed, the input resistors may be adjusted to the
desired gain, but also change the input impedance as
well. The high-frequency common-mode gain for this
circuit from input to output is the same as for the
signal gain. Again, if the source might include an
undesired common-mode signal, that could be
rejected at the input using blocking caps (for
low-frequency and dc common-mode) or a
transformer coupling. The differential performance
plots shown in the Typical Characteristics used the
configuration of Figure 75 and an input 1:1
transformer. The differential signal gain in the circuit
of Figure 75 is:
A_D = R_F/R_G
Using this configuration suppresses the second
harmonics, leaving only third harmonic terms as the
limit to output SFDR. The much higher slew rate of
the inverting configuration also extends the full-power
bandwidth and the range of very low intermodulation
distortion over the performance bandwidth available
from the circuit of Figure 74. The Typical
Characteristics show that the circuit of Figure 75
operating at an A_D = 10 can deliver a 16V_PP signal
with over 400MHz –3dB bandwidth. Using
Equation 3, this implies a differential output slew of
18000V/µs, or 9000V/µs at each output. This output
slew rate is far higher than specified, and probably
due to the lighter load used in the differential tests.
F_MAX =
Slew Rate
2pV_P (0.707)
(3)
This inverting input differential configuration is
particularly suited to very high SFDR converter
interfaces—specifically narrowband IF channels. The
Typical Characteristics show the two-tone, third-order
intermodulation intercept exceeding 45dBm through
90MHz. Although this data was taken with an 800Ω
load, the intercept model appears to work for this
circuit, simply treating the power level as if it were
into 50Ω. For example, at 70MHz, the differential
Typical Characteristic plots show a 48dBm intercept.
To predict the two-tone intermodulation SFDR,
assuming a –1dB below full-scale envelope to a 2V_PP
maximum differential input converter, the test power
level would be 9dBm – 6dBm = 3dBm for each tone.
Putting this into the intercept equation, gives:
ΔdBc = 2 × (48 – 3) = 90dBc
The single-tone distortion data shows approximately
72dB SFDR at 70MHz for a 2V_PP output into this light
800Ω load. A modest post filter after the amplifier can
reduce these harmonics (second at 140MHz, third at
210MHz) to the point where the full SFDR to a
converter can be in the 85dB range for a 70MHz IF
operation.
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