OPA2822UG4 Datasheet, PDF (17/33 Pages) Texas Instruments – Dual, Wideband, Low-Noise Operational Amplifier

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.
+V_S
+5V
365Ω
2.2µF 2.2µF
1/2
OPA2822
2kΩ
730Ω
1µF
V_I
V_S
2
V_O
2kΩ
730Ω
2.2µF 2.2µF
1/2
OPA2822
365Ω
FIGURE 8. Single-Supply, 2nd-Order High-Pass Active
Filter with Differential I/O.
2.2pF
140Ω 2.1kΩ
V_I
1.0nF 1.0nF
1/2
1.3kΩ _OPA2822
143Ω
158Ω 225Ω
150pF 12pF
180pF
+5V
18pF
1/2
OPA2822
107Ω
–5V
1.8nF 300Ω
150Ω
V_O
66pF
FIGURE 6. 6th-Order Bandpass Filter.
10
0
–10
–20
–30
–40
–50
–60
1.0E+04
1.0E+05
1.0E+06
1.0E+07
Frequency (Hz)
1.0E+08
FIGURE 7. Frequency Response for the Filter in Figure 6.
3
0
–3
–6
–9
–12
–15
–18
–21
–24
–27
–30
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
1.0E+07
FIGURE 9. Frequency Response for the Filter in Figure 8.
OPA2822
17
SBOS188E
www.ti.com