OPA2604 Datasheet, PDF (15/26 Pages) Burr-Brown (TI) – Dual FET-Input, Low Distortion OPERATIONAL AMPLIFIER

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Typical Applications (continued)
8.2.1.3 Application Curve
20
OPA2604
SBOS006A – SEPTEMBER 2000 – REVISED DECEMBER 2015
0
-20
-40
-60
100
1k
10k
100k
1M
Frequency (Hz)
Figure 25. 25-kHz Low Pass Filter Response
8.2.2 Three-Pole Generalized-Immittance Converter (GIC) Low-Pass Filter
In any digitizing system, anti-aliasing and anti-imaging filters are used to prevent the signal frequencies from
folding back around the sample frequency and causing false (or alias) signals from appearing in the signal we
are attempting to digitize. Very often, these filters must be very complex, high order analog filters to do their job
effectively.
The filter characteristic most desirable for sensitive DSP type applications is linear-phase. The linear-phase filter
is sometimes called a Bessel (or Thomson) filter. The linear-phase filter has constant group delay. This means
that the phase of the filter changes linearly with frequency, or that the group delay is constant. These filters
maintain phase information for sensitive DSP applications such as correlation, and preserve transient response.
These characteristics are critical in audio applications as well, because they affect sound quality greatly.
Illustrated in Figure 26 is a third-order low pass filter with 40-kHz cutoff frequency designed for audio
applications.
3.92 k
1.33 k
Input ⁺
±
3.92 k
3.92 k
±
+
½ OPA2604
½ OPA2604
+
±
1000 pF
±
+
1000 pF
OPA627
Output
3.48 k
1000 pF
Figure 26. Three-Pole Generalized-Immittance Converter (GIC) Low-Pass Filter
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