OPA4830 Datasheet, PDF (25/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

OPA4830
www.ti.com.................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008
NONINVERTING AMPLIFIER WITH REDUCED
PEAKING
Figure 78 shows a noninverting amplifier that reduces
peaking at low gains. The resistor RC compensates
the OPA4830 to have higher noise gain (NG), which
reduces the ac response peaking (typically 5dB at G
= +1V/V without RC) without changing the dc gain.
V_IN needs to be a low-impedance source, such as an
op amp. The resistor values are low in order to
reduce noise. Using both R_T and R_F helps minimize
the impact of parasitic impedances.
R_T
V_IN
+5V
1/4
R_{C OPA4830}
V_OUT
R_G
R_F
Figure 78. Compensated Noninverting Amplifier
The noise gain can be calculated as shown in
Equation 3, Equation 4, and Equation 5:
G₁ = 1 +
R_F
R_G
(3)
G₂ = 1 +
R_T +
R_F
G₁
R_C
(4)
NG = G₁ ´ G₂
(5)
A unity-gain buffer can be designed by selecting R_T =
R_F = 20.0Ω and R_C = 40.2Ω (do not use R_G). This
circuit gives a noise gain of 2V/V, so the response is
similar to the characteristics plots with G = +2V/V.
Decreasing R_C to 20.0Ω increases the noise gain to
3V/V, which typically gives a flat frequency response,
but with less bandwidth.
The circuit in Figure 72 can be redesigned to have
less peaking by increasing the noise gain to 3. This
increase is accomplished by adding R_C = 2.55kΩ
across the op amp inputs.
SINGLE-SUPPLY ACTIVE FILTER
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 0.1µF blocking capacitors (actually
giving bandpass response with the low-frequency
pole set to 32kHz for the component values shown).
As discussed for Figure 72, this configuration 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 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
maximum stop band attenuation of 36dB at the
amplifier –3dB bandwidth of 30MHz.
0.1mF
V_I
1.87kW
137W
1.87kW
+5V
100pF
432W
1/4
150pF OPA4830
1.5kW
4V_I
1MHz, 2nd-Order
Butterworth Filter
500W
0.1mF
Figure 79. Single-Supply, High-Frequency Active Filter
Copyright © 2006–2008, Texas Instruments Incorporated
Product Folder Link(s): OPA4830
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