OPA830 datasheet(25/45 Pages) TI

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NONINVERTING AMPLIFIER WITH REDUCED

SINGLE-SUPPLY ACTIVE FILTER

1/4

OPA4830

OUT

+5V

G =1+

(3)

G =1+

T +

(4)

NG=G

(5)

1/4

OPA4830

1.5kW

1.87kW

500W

+5V

1.87kW

150pF

0.1 F

1MHz,2nd-Order

ButterworthFilter

100pF

432W

137W

0.1 F

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 RC to 20.0Ω 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

VIN needs to be a low-impedance source, such as an

increase is accomplished by adding RC = 2.55kΩ

op amp. The resistor values are low in order to

across the op amp inputs.

reduce noise. Using both RT and RF 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

µF 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 unity-gain buffer can be designed by selecting RT =

maximum stop band attenuation of 36dB at the

RF = 20.0Ω and RC = 40.2Ω (do not use RG). This

amplifier –3dB bandwidth of 30MHz.

Figure 79. Single-Supply, High-Frequency Active Filter

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