Datasheet

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SBOS263F − AUGUST 2004 − REVISED AUGUST 2008
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22
SINGLE-SUPPLY ACTIVE FILTER
The OPA830, while operating on a single +3V or +5V
supply, lends itself well to high-frequency active filter
designs. Again, the key additional requirement is to
establish the DC operating point of the signal near the
supply midpoint for highest dynamic range. Figure 8
shows an example design of a 1MHz low-pass Butterworth
filter using the Sallen-Key topology.
Both the input signal and the gain setting resistor are
AC-coupled using 0.1µF blocking capacitors (actually
giving bandpass response with the low-frequency pole set
to 32kHz for the component values shown). As discussed
for Figure 1, this allows the midpoint bias formed by the
two 1.87kΩ resistors to appear at both the input and output
pins. The midband signal gain is set to +4 (12dB) in this
case. The capacitor to ground on the noninverting input is
intentionally set larger to dominate input parasitic terms. At
a gain of +4, the OPA830 on a single supply will show
30MHz small- and large-signal bandwidth. The resistor
values have been slightly adjusted to account for this
limited bandwidth in the amplifier stage. Tests of this circuit
show a precise 1MHz, −3dB point with a maximally-flat
passband (above the 32kHz AC-coupling corner), and a
maximum stop band attenuation of 36dB at the amplifier’s
−3dB bandwidth of 30MHz.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two printed circuit boards (PCBs) are available to assist
in the initial evaluation of circuit performance using the
OPA830 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 1.
Table 1. Demonstration Fixtures by Package
ORDERING LITERATURE
PRODUCT PACKAGE
ORDERING
NUMBER
LITERATURE
NUMBER
OPA830ID SO-8
DEM-OPA-SO-1A
SBOU009
OPA830IDBV SOT23-5
DEM-OPA-SOT-1A
SBOU010
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA830
product folder.
OPA830
1.5k
Ω
1.87k
Ω
500
Ω
+5V
1.87k
Ω
4V
I
V
I
150pF
0.1
µ
F
1MHz, 2nd−Order
Butterworth Filter
100pF
432
Ω
137
Ω
0.1
µ
F
Figure 8. Single-Supply, High-Frequency Active Filter