OPA2694ID Datasheet, PDF (15/28 Pages) Texas Instruments – Dual, Wideband, Low-Power, Current Feedback Operational Amplifier

OPA2694
www.ti.com
DIFFERENTIAL ACTIVE FILTER
The OPA2694 can provide a very capable gain block for
low-noise active filters. The dual design lends itself very
well to differential active filters. Where the filter topology is
looking for a simple gain function to implement the filter,
the noninverting configuration is preferred to isolate the
filter elements from the gain elements in the design.
Figure 9 shows an example of a very low power, 10MHz
3rd-order Butterworth low-pass, Sallen-Key filter. The
example of Figure 9 designs the filter for a differential gain
of 1 using the OPA2694. The resistor values have been
adjusted slightly to account for the amplifier bandwidth
effects.
While this circuit is bipolar (using ±5V supplies), it can
easily be adapted to single-supply operation. This is
typically done by providing a supply midpoint reference at
the noninverting inputs, and then adding DC blocking caps
at each input and in series with the amplifier gain resistor,
R_G. This will add two real zeroes in the response,
transforming the circuit into a bandpass.
50Ω 232Ω
100pF
20Ω
+5V
1/2
OPA2694
800Ω 357Ω
V_I
75pF 400Ω _800Ω
357Ω
22pF V_O
50Ω 232Ω
1/2
20Ω OPA2694
100pF
−5V
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist
in the initial evaluation of circuit performance using the
OPA2694 in either of two possible configurations: inverting
or noninverting. 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 1.
Table 1. Demonstration Fixtures by Package
PRODUCT PACKAGE
OPA2694ID
SO-8
OPA2694ID
SO-8
ORDERING
NUMBER
DEM-OPA-SO-2B
(noninverting)
DEM-OPA-SO-2C
(inverting)
LITERATURE
NUMBER
SBOU030
SBOU029
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2694
product folder.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE
is often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and
inductance can have a major effect on circuit performance.
A SPICE model for the OPA2694 is available through the
TI web site (www.ti.com). These models do a good job of
predicting small-signal AC and transient performance
under a wide variety of operating conditions. They do not
do as well in predicting the harmonic distortion or dG/dφ
characteristics. These models do not attempt to
distinguish between package types in their small-signal
AC performance.
Figure 9. Low-Power, Differential I/O, 3rd-Order
Butterworth Active Filter
15