OPA695IDGKR Datasheet, PDF (23/41 Pages) Texas Instruments – Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

noninverting inputs again have a gain of 1 to the output pins,
giving particularly easy common-mode control for single-
supply operation. The OPA695 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 will also be changing the
input impedance as well. The high-frequency common-mode
gain for this circuit from input to output will be 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 14 and an input 1:1 transformer.
The differential signal gain in the circuit of Figure 14 is:
A_D = R_F /R_G
(7)
Using this configuration suppresses the 2nd-harmonics, leav-
ing only 3rd-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 13. The Typical Charac-
teristics show that the circuit of Figure 14 operating at an
A_D = 10 can deliver a 16V_PP signal with over 500MHz –3dB
bandwidth. Using Equation 4, this implies a differential output
slew of 18000V/µsec, or 9000V/µsec at each output. This
output slew rate is far higher than specified, and probably
due to the lighter load used in the differential tests.
This inverting input differential configuration is particularly
suited to very high SFDR converter interfaces—specifically
narrowband IF channels. The Typical Characteristics show
the 2-tone, 3rd-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 2-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:
∆dBc = 2 • (48 – 3) = 90dBc
(8)
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 (2nd at 140MHz, 3rd at 210MHz) to the point
where the full SFDR to a converter can be in the 85dB range
for a 70MHz IF operation.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA695
in its two package options. Both of these are offered free of
charge as unpopulated PCBs, delivered with a user's guide.
The summary information for these fixtures is shown in the
table below.
PRODUCT
OPA695ID
OPA691IDBV
PACKAGE
SO-8
SOT23-6
ORDERING
NUMBER
DEM-OPA-SO-1B
DEM-OPA-SOT-1B
LITERATURE
NUMBER
SBOU026
SBOU027
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA695
product folder.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE
BANDWIDTH
A current-feedback op amp such as the OPA695 can hold an
almost constant bandwidth over signal gain settings with the
proper adjustment of the external resistor values. This is
shown in the Typical Characteristic curves. The small-signal
bandwidth decreases only slightly with increasing gain. These
curves also show that the feedback resistor has been changed
for each gain setting. The resistor values on the inverting
side of the circuit for a current-feedback op amp can be
treated as frequency response compensation elements while
their ratios set the signal gain. Figure 15 shows the analysis
circuit for the OPA695 small-signal frequency response.
The key elements of this current feedback op amp model are:
α ⇒ Buffer gain from the noninverting input to the invert-
ing input
R_I ⇒ Buffer output impedance
i_ERR ⇒ Feedback error current signal
Z(s) ⇒ Frequency-dependent, open-loop transimpedance
gain from i_ERR to V_O
V_I
i_ERR
α
R_I
R_F
V_O
Z_(S) i_ERR
R_G
OPA695
SBOS293G
FIGURE 15. Current-Feedback Transfer Function Analysis
Circuit.
23
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