OPA2690 datasheet(17/33 Pages) TI

17 / 33 page

OPA2822

SBOS188E

www.ti.com

ACTIVE FILTER APPLICATIONS

As a low-noise, low-distortion, unity-gain stable, voltage-

feedback amplifier, the OPA2822 provides an ideal building

block for high-performance active filters. With two channels

available, it can be used either as a cascaded 2-stage active

filter or as a differential filter. Figure 6 shows a 6th-order

bandpass filter cascaded with two 2nd-order Sallen-Key

sections, with transmission zeroes along with a passive post

filter made up of a high-pass and a low-pass section. The first

amplifier provides a 2nd-order high-pass stage while the

second amplifier provides the 2nd-order low-pass stage.

Figure 7 shows the frequency response for this example

filter.

A differential active filter is shown in Figure 8. This circuit

shows a single-supply, 2nd-order high-pass filter with the

corner frequencies set to provide the required high-pass

function for an ADSL CPE modem application. To use this

circuit, the hybrid would be implemented as a passive sum-

ming circuit at the input to this filter. For +5V only ADSL

designs, it is preferable to implement a portion of the filtering

prior to the amplifier, thus limiting the amplitude of the

uncancelled line driver signals. This type of receiver stage

would typically then drive a low-pass filter prior to the codec

setting the high-frequency cutoff of the ADC (Analog-to-

Digital Converter) input signal. Figure 9 shows the frequency

response for the high-pass circuit of Figure 8.

FIGURE 6. 6th-Order Bandpass Filter.

FIGURE 7. Frequency Response for the Filter in Figure 6.

FIGURE 9. Frequency Response for the Filter in Figure 8.

FIGURE 8. Single-Supply, 2nd-Order High-Pass Active

Filter with Differential I/O.

–5V

1/2

OPA2822

1/2

OPA2822

+5V

225

Ω

158

Ω

150pF

12pF

18pF

180pF

66pF

150

Ω

107

Ω

2.1k

Ω

140

Ω

1.0nF

2.2pF

143

Ω

300

Ω

1.8nF

1.3k

Ω

–10

–20

–30

–40

–50

–60

Frequency (Hz)

1.0E+04

1.0E+05

1.0E+06

1.0E+08

1.0E+07

1/2

OPA2822

+5V

1/2

OPA2822

2.2

µF

2.2

µF

2.2

µF

2.2

µF

365

Ω

365

Ω

730

Ω

730

Ω

µF

–3

–6

–9

–12

–15

–18

–21

–24

–27

–30

Frequency (Hz)

1.0E+04

1.0E+05

1.0E+06

1.0E+07

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