Datasheet

OPA694
+5V
R
O
50W
V
O
R
F
430W
R
G
430W
R
M
50W
-5V
V
I
OPA694
SAW
Filter
+12V
Matching
Network
=12dB (SAWLoss)-
50W
Source
50W
50W
P
O
P
O
P
I
P
I
400W
50W
0.1 Fm1000pF
1000pF
5kW
5kW
3
0
-3
-6
-9
-12
NormalizedGain(dB)
10M 100M 3G1G
Frequency(Hz)
G=+1,Figure1
G=+1,Figure6
OPA694
SBOS319G –SEPTEMBER 2004–REVISED JANUARY 2010 www.ti.com
SAW FILTER BUFFER small-signal frequency response for the unity gain
buffer of Figure 31 compared to the improved
One common requirement in an IF strip is to buffer
approach shown in Figure 36. Either approach gives
the output of a mixer with enough gain to recover the
a low-power unity-gain buffer with > 1.56GHz
insertion loss of a narrowband SAW filter. Figure 35
bandwidth.
shows one possible configuration driving a SAW filter.
The Two-Tone, Third-Order Intermodulation Intercept
plot (Figure 14) is shown in the Typical
Characteristics 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 OPA694 in this
Figure 36. Improve Unity Gain Buffer
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.
Figure 35. IF Amplifier Driving SAW Filter
Figure 37. Gain of +1 Frequency Response
WIDEBAND UNITY GAIN BUFFER WITH
IMPROVED FLATNESS
DESIGN-IN TOOLS
The unity gain buffer configuration of Figure 31
shows a peaking in the frequency response
DEMONSTRATION FIXTURES
exceeding 2dB. This gives the slight amount of
overshoot and ringing apparent in the gain of +1V/V
Two printed circuit boards (PCBs) are available to
pulse response curves. A similar circuit that holds a
assist in the initial evaluation of circuit performance
flatter frequency response, giving improved pulse
using the OPA694 in its two package options. Both of
fidelity, is shown in Figure 36.
these are offered free of charge as unpopulated
PCBs, delivered with a user’s guide. The summary
This circuit removes the peaking by bootstrapping out
information for these fixtures is shown in Table 1.
any parasitic effects on R
G
. The input impedance is
still set by R
M
as the apparent impedance looking into
Table 1. Demonstration Fixtures by Package
R
G
is very high. R
M
may be increased to show a
higher input impedance, but larger values will start to
ORDERING LITERATURE
impact DC output offset voltage. This circuit creates
PRODUCT PACKAGE NUMBER NUMBER
an additional input offset voltage as the difference in
DEM-OPA-
OPA694ID SO-8 SBOU026
the two input bias currents times the impedance to
SO-1B
ground at V
I
. Figure 37 shows a comparison of
DEM-OPA-
OPA694IDBV SOT23-5 SBOU027
SOT-1B
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