
14
OPA3680
FIGURE 8. State Variable Filter.
BOARD
PART
NUMBER
LITERATURE
REQUEST
NUMBER
PRODUCT
PACKAGE
OPA3680E
OPA3680U
16-Lead SSOP
SO-16
DEM-OPA368xE
DEM-OPA368xU
MKT-354
MKT-364
STATE VARIABLE FILTER
Figure 8 shows a state variable filter using the OPA3680.
This active filter is quite useful for high Q filter responses,
and will produce lowpass, highpass, bandpass, notch and
allpass functions. The filter response is:
(2)
The desired filter frequency response is achieved by the
correct selection of the feed-forward components at the
input.
The resistor R
ISO
isolates the last op amp and the input
driver from capacitive loading problems when
α
> 0. To
ensure good performance, make sure that:
(3)
where, f
GBP
is the OPA3680’s gain bandwidth product
(300MHz).
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
PC boards are available to assist in the initial evaluation of
circuit performance using the OPA3680. They are available
free as an unpopulated PC board delivered with descriptive
documentation. The summary information for the boards is
shown below:
Contact the Burr-Brown Applications support line
(1-800-548-6132) to request this board (ask for the desired
literature number).
MACROMODELS
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 induc-
tance can have a major effect on circuit performance. A
SPICE model for the OPA680 is available through either the
Burr-Brown Internet web page (http://www.burr-brown.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 har-
monic distortion, temperature performance or dG/d
φ
charac-
teristics. These models do not attempt to distinguish be-
tween the package types in their small-signal AC perfor-
mance.
V
IN
V
OUT
R/
δ
C
49.9
R
R
R
R
C
Q
P
R
Q
P
R/
χ
Q
P
R/
β
R
ISO
1/3
OPA3680
1/3
OPA3680
α
C
49.9
49.9
1/3
OPA3680
ω
2
π
P
GBP
Q
20
P
P
GBP
20
P
f
Q
f
Q
1
1
≤
>
≤
,
,
V
V
s
s
Q
s
s
Q
RC
OUT
IN
P
P
P
P
P
P
P
=
( )
+
α
(
ω
)
+
( )
δ
+
+
=
–
–
2
2
2
2
1
ω
β
χ
ω
ω
ω