O PA820

+5V

−5V

R

2

505Ω

C

1

150pF

R

1

124Ω

V

V

O

1

R

G

402Ω

R

F

402Ω

C

2

100pF

Power−supply

decoupling not shown.

+

=

+

+

3

4

OUT

1

4

1

3

IN

1

1

2

4

5

1

2

2

R

R

S

V

R R C

1

R

V

S

S

R C

R R R C C

2

O

w

=

3

2

4

5

1

2

R

R R R C C

1 1

1

O

Q

R C

w

=

w

=

p

;

O

O

f

1MHz

2

OPA820-HT

SBOS587

– DECEMBER 2011

www.ti.com

Active Filters

Most active filter topologies will have exceptional performance using the broad bandwidth and unity-gain stability

of the OPA820. Topologies employing capacitive feedback require a unity-gain stable, voltage-feedback op amp.

Sallen-Key filters simply use the operational amplifier as a noninverting gain stage inside an RC network. Either

current- or voltage-feedback op amps may be used in Sallen-Key implementations.

Figure 9 shows an example Sallen-Key low-pass filter, in which the OPA820 is set up to deliver a low-frequency

gain of +2. The filter component values have been selected to achieve a maximally-flat Butterworth response

with a 5-MHz,

−3-dB bandwidth. The resistor values have been slightly adjusted to compensate for the effects of

the 240-MHz bandwidth provided by the OPA820 in this configuration. This filter may be combined with the ADC

driver suggestions to provide moderate (2-pole) Nyquist filtering, limiting noise, and out-of-band harmonics into

the input of an ADC. This filter will deliver the exceptionally low harmonic distortion required by high SFDR ADCs

such as the ADS850 (14-bit, 10 MSPS, 82-dB SFDR).

Figure 9. 5-MHz Butterworth Low-Pass Active Filter

Another type of filter, a high-Q bandpass filter, is shown in Figure 10. The transfer function for this filter is:

(3)

with

(4)

and

(5)

For the values chosen in Figure 10:

(6)

and Q = 100.

See Figure 11 for the frequency response of the filter shown in Figure 10.

22

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© 2011, Texas Instruments Incorporated

Product Folder Link(s): OPA820-HT