OPA830 datasheet(25/43 Pages) TI1

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NONINVERTING AMPLIFIER WITH REDUCED

SINGLE-SUPPLY ACTIVE FILTER

1/2

OPA2830

OUT

+5V

+ 1 )

)

NG + G1

1/2

OPA2830

1.5k

Ω

1.87k

Ω

500

Ω

+5V

1.87k

Ω

150pF

0.1

µF

1MHz, 2nd−Order

Butterworth Filter

100pF

432

Ω

137

Ω

0.1

µF

OPA2830

www.ti.com.................................................................................................................................................. SBOS309D – AUGUST 2004 – REVISED AUGUST 2008

PEAKING

The OPA2830 operating on a single +3V or +5V

Figure 77 shows a noninverting amplifier that reduces

supply lends itself well to high-frequency active filter

peaking at low gains. The resistor RC compensates

designs.

The

key

additional

requirement

the OPA2830 to have higher Noise Gain (NG), which

establish the DC operating point of the signal near

reduces the AC response peaking (typically 4dB at

the supply midpoint for highest dynamic range.

G = +1 without RC) without changing the DC gain. VIN

Figure 78 shows an example design of a 1MHz

needs to be a low impedance source, such as an op

low-pass Butterworth filter using the Sallen-Key

amp.

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).

This allows the midpoint bias formed by the two

1.87k

Ω 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 designed at a higher

value to dominate input parasitic terms. At a gain of

+4, the OPA2830 on a single supply will show 30MHz

small- and large-signal bandwidth. The filter resistor

values have been slightly adjusted to account for this

Figure 77. Compensated Noninverting Amplifier

limited bandwidth in the amplifier stage. Tests of this

circuit show a precise 1MHz, –3dB point with a

The Noise Gain can be calculated as follows:

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.

A unity-gain buffer can be designed by selecting

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

This gives a noise gain of 2, so the response will be

similar to the Characteristics Plots with G = +2 giving

less peaking.

Figure 78. Single-Supply, High-Frequency Active Filter

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