Datasheet

OPA2822
17
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
V
O
V
I
1/2
OPA2822
1/2
OPA2822
+5V
225158
150pF 12pF
18pF
180pF
66pF150
107
2.1k140
1.0nF 1.0nF
2.2pF
143
3001.8nF
1.3k
10
0
10
20
30
40
50
60
Frequency (Hz)
1.0E+04 1.0E+05 1.0E+06 1.0E+081.0E+07
Gain (dB)
1/2
OPA2822
+5V
1/2
OPA2822
V
I
V
O
+V
S
V
S
2
2.2µF 2.2µF
2.2µF 2.2µF
365
365
2k
2k
730
730
1µF
3
0
3
6
9
12
15
18
21
24
27
30
Frequency (Hz)
1.0E+04 1.0E+05 1.0E+06 1.0E+07
Gain (dB)