Datasheet

OPA684
15
SBOS219D
www.ti.com
Figure 8 shows the small-signal frequency response for the
amplifier portion of this circuit, from the non-inverting input to
the output pin. This particular example is configured as a
single-supply circuit that will operate over a supply range from
+5V to +12V. Where higher frequencies or 3rd-order intercepts
are required, consider the OPA685, a very high bandwidth
(> 500MHz) current-feedback op amp.
the non-inverting converter input and the amplifier non-
inverting input. With an AC-coupled gain path, this +2.5V has
a gain of +1 to the output, putting the output at the DC
midpoint for the converter. The output then drives through an
isolating resistor (50Ω) to the inverting input of the converter,
which is further de-coupled by a 10pF external capacitance
to add to its 5pF input capacitance. This coupling network
provides a high frequency cutoff, while also giving a low
source impedance at high frequencies for the converter. The
gain for this circuit is set by adjusting R
G
to the desired value.
For a 2V
PP
maximum output driving the light load of Figure
9, the OPA684 will provide > 75dB SFDR through 5MHz (see
the Typical Characteristics).
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA684
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
Table I.
30
27
24
21
18
15
Frequency (MHz)
10 100
BANDPASS SAW FILTER POST AMPLIFIER
Gain (dB)
44MHz
LOW POWER, ADC DRIVER
Where a low-power, single-supply, interface to a single-
ended input +5V ADC is required, the circuit of Figure 9 can
provide a very flexible, higher performance solution. Running
in an AC-coupled inverting mode allows the non-inverting
input to be used for the CM (Common-Mode) voltage from
the ADS825 converter. This midpoint reference biases both
FIGURE 8. Bandpass SAW Filter Post-Amplifier.
FIGURE 9. Low-Power, Single-Supply, ADC Driver.
OPA684
+5V
DIS
50Ω
R
G
R
G
R
I
1.3kΩ
50Ω
10pF
V
I
V
O
1.3kΩ
0.1µF
V
O
= – V
I
IN
IN
CM
ADS825
10-Bit
40MSPS
2.5V
DC
+2.5V
2Vp-p
Max
ORDERING LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA684ID SO-8 DEM-OPA-SO-1A SBOU009
OPA684IDBQ SOT23-6 DEM-OPA-SOT-1A SBOU010
TABLE I. Demonstration Fixtures by Package.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA684
product folder.