OPA2694ID Datasheet, PDF (13/28 Pages) Texas Instruments – Dual, Wideband, Low-Power, Current Feedback Operational Amplifier

OPA2694
www.ti.com
SAW FILTER BUFFER
One common requirement in an IF strip is to buffer the
output of a mixer with enough gain to recover the insertion
loss of a narrowband SAW filter. Figure 5 shows one
possible configuration driving a SAW filter. The 2-Tone,
3rd-Order Intermodulation Intercept plot is shown in the
Typical Characteritics curves. Operating in the inverting
mode at a voltage gain of –8V/V, this circuit provides a 50Ω
input match using the gain set resistor, has the feedback
optimized for maximum bandwidth (250MHz in this case),
and drives through a 50Ω output resistor into the matching
network at the input of the SAW filter. If the SAW filter gives
a 12dB insertion loss, a net gain of 0dB to the 50Ω load at
the output of the SAW (which could be the input
impedance of the next IF amplifier or mixer) will be
delivered in the passband of the SAW filter. Using the
OPA2694 in this application will isolate the first mixer from
the impedance of the SAW filter and provide very low
two-tone, 3rd-order spurious levels in the SAW filter
bandwidth.
+12V
1000pF
5kΩ
0.1μF
1/2
5kΩ OPA2694
50Ω
Source _{1000pF 50Ω}
P_I
400Ω
50Ω
Matching
N e two rk
SAW
Filter
P_O
50Ω
P_{O = 12dB − (SAW Loss)}
P_I
Figure 5. IF Amplifier Driving SAW Filter
DIFFERENTIAL INTERFACE APPLICATIONS
Dual op amps are particularly suitable to differential input
to differential output applications. Typically, these fall into
either ADC input interface or line driver applications. Two
basic approaches to differential I/O are noninverting or
inverting configurations. Since the output is differential, the
signal polarity is somewhat meaningless—the
noninverting and inverting terminology applies here to
where the input is brought into the OPA2694. Each has its
advantages and disadvantages. Figure 6 shows a basic
starting point for noninverting differential I/O applications.
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
+V_CC
1/2
OPA2694
R_F
V_I
R_G
R_F
V_O
1/2
OPA2694
−V_CC
Figure 6. Noninverting Differential I/O Amplifier
This approach provides for a source termination
impedance that is independent of the signal gain. For
instance, simple differential filters may be included in the
signal path right up to the noninverting inputs without
interacting with the gain setting. The differential signal gain
for the circuit of Figure 6 is shown in Equation (1):
A_D + 1 ) 2
R_F
R_G
(1)
The differential gain, however, may be adjusted with
considerable freedom using just the R_G resistor. In fact, R_G
may be a reactive network providing a very isolated
shaping to the differential frequency response. Since the
inverting inputs of the OPA2694 are low-impedance
closed-loop buffer outputs, the R_G element does not
interact with the amplifier bandwidth. Wide ranges of
resistor values and/or filter elements may be inserted here
with minimal amplifier bandwidth interaction.
Various combinations of single-supply or AC-coupled gain
can also be delivered using the basic circuit of Figure 6.
Common-mode bias voltages on the two noninverting
inputs pass on to the output with a gain of 1, since an equal
DC voltage at each inverting node creates no current
through R_G. This circuit does show a common-mode gain
of 1 from input to output. The source connection should
either remove this common-mode signal if undesired
(using an input transformer can provide this function), or
the common-mode voltage at the inputs can be used to set
the output common-mode bias. If the low common-mode
rejection of this circuit is a problem, the output interface
may also be used to reject that common-mode. For
instance, most modern differential input ADCs reject
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