Datasheet
NONINVERTING AMPLIFIER WITH REDUCED
SINGLE-SUPPLY ACTIVE FILTER
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OPA4830
V
OUT
+5V
V
IN
R
G
R
T
R
F
R
C
R
F
R
G
G =1+
1
(3)
R
C
G =1+
2
R
T
+
R
F
G
1
(4)
NG=G G´
1 2
(5)
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OPA4830
1.5kW
1.87kW
500W
+5V
1.87kW
4V
I
V
I
150pF
0.1 Fm
1MHz,2nd-Order
ButterworthFilter
100pF
432W137W
0.1 Fm
OPA4830
www.ti.com
.................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008
circuit gives a noise gain of 2V/V, so the response is
PEAKING similar to the characteristics plots with G = +2V/V.
Decreasing R
C
to 20.0 Ω increases the noise gain to
Figure 78 shows a noninverting amplifier that reduces
3V/V, which typically gives a flat frequency response,
peaking at low gains. The resistor RC compensates
but with less bandwidth.
the OPA4830 to have higher noise gain (NG), which
reduces the ac response peaking (typically 5dB at G The circuit in Figure 72 can be redesigned to have
= +1V/V without RC) without changing the dc gain. less peaking by increasing the noise gain to 3. This
V
IN
needs to be a low-impedance source, such as an increase is accomplished by adding R
C
= 2.55k Ω
op amp. The resistor values are low in order to across the op amp inputs.
reduce noise. Using both R
T
and R
F
helps minimize
the impact of parasitic impedances.
The OPA4830, 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 79 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 0.1 µ F blocking capacitors (actually
giving bandpass response with the low-frequency
pole set to 32kHz for the component values shown).
Figure 78. Compensated Noninverting Amplifier
As discussed for Figure 72 , this configuration allows
the midpoint bias formed by the two 1.87k Ω resistors
The noise gain can be calculated as shown in
to appear at both the input and output pins. The
Equation 3 , Equation 4 , and Equation 5 :
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 OPA4830 on a single
supply shows 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
A unity-gain buffer can be designed by selecting R
T
=
maximum stop band attenuation of 36dB at the
R
F
= 20.0 Ω and R
C
= 40.2 Ω (do not use R
G
). This
amplifier – 3dB bandwidth of 30MHz.
Figure 79. Single-Supply, High-Frequency Active Filter
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