Datasheet

F =
MAX
SlewRate
2 V (0.707)p
P
(3)
OPA2695
www.ti.com ...................................................................................................................................................... SBOS354A – APRIL 2008 – REVISED AUGUST 2008
The two noninverting inputs provide an easy with over 400MHz – 3dB bandwidth. Using
common-mode control input. This is particularly easy Equation 3 , this implies a differential output slew of
if the source is ac-coupled through either blocking 18000V/ µ s, or 9000V/ µ s at each output. This output
caps or a transformer. In either case, the slew rate is far higher than specified, and probably
common-mode input voltages on the two noninverting due to the lighter load used in the differential tests.
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
This inverting input differential configuration is
500 Ω region for best frequency response. With R
F particularly suited to very high SFDR converter
fixed, the input resistors may be adjusted to the
interfaces — specifically narrowband IF channels. The
desired gain, but also change the input impedance as
Typical Characteristics show the two-tone, third-order
well. The high-frequency common-mode gain for this
intermodulation intercept exceeding 45dBm through
circuit from input to output is the same as for the
90MHz. Although this data was taken with an 800 Ω
signal gain. Again, if the source might include an
load, the intercept model appears to work for this
undesired common-mode signal, that could be
circuit, simply treating the power level as if it were
rejected at the input using blocking caps (for
into 50 Ω . For example, at 70MHz, the differential
low-frequency and dc common-mode) or a
Typical Characteristic plots show a 48dBm intercept.
transformer coupling. The differential performance
To predict the two-tone intermodulation SFDR,
plots shown in the Typical Characteristics used the
assuming a – 1dB below full-scale envelope to a 2V
PP
configuration of Figure 75 and an input 1:1
maximum differential input converter, the test power
transformer. The differential signal gain in the circuit
level would be 9dBm – 6dBm = 3dBm for each tone.
of Figure 75 is:
Putting this into the intercept equation, gives:
A
D
= R
F
/R
G
Δ dBc = 2 × (48 – 3) = 90dBc
Using this configuration suppresses the second
The single-tone distortion data shows approximately
harmonics, leaving only third harmonic terms as the
72dB SFDR at 70MHz for a 2V
PP
output into this light
limit to output SFDR. The much higher slew rate of
800 Ω load. A modest post filter after the amplifier can
the inverting configuration also extends the full-power
reduce these harmonics (second at 140MHz, third at
bandwidth and the range of very low intermodulation
210MHz) to the point where the full SFDR to a
distortion over the performance bandwidth available
converter can be in the 85dB range for a 70MHz IF
from the circuit of Figure 74 . The Typical
operation.
Characteristics show that the circuit of Figure 75
operating at an A
D
= 10 can deliver a 16V
PP
signal
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