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Maxim > Design Support > Technical Documents > Application Notes > Filter Circuits (Analog) > APP 1028
Maxim > Design Support > Technical Documents > Application Notes > Video Circuits > APP 1028
Keywords: 3-pole, butterworth, active filter, low pass filter, video, output, NTSC, PAL, reconstruction
filtering, operational amplifier, op amp, opamps
APPLICATION NOTE 1028
SDTV Reconstruction Filter with Adjustable Group
Delay
Sep 30, 2002
Abstract: The MAX4450 operational amplifier configured in a 3-pole, low-pass Sallen-Key filter provides
a Butterworth response with a bandwidth of -3dB at 5.25MHz. This circuit can be used for video anti-
aliasing and reconstruction filtering for composite (CVBS) or S-Video signals in standard definition digital
TV (SDTV) applications. The circuit is designed to drive a 75Ω termination, common in video
applications, with an overall gain of 1.
This application note describes a 5.25MHz, 3-Pole, Butterworth filter using a Sallen-Key realization. It
has gain of 2V/V, for driving a 75Ω back-terminated coax to an overall gain of 1. Such filters are used for
video reconstruction of RGB and component video (Y, Pb, Pr) signals following a DAC to remove the
higher frequency replicas of the signal, and as an anti-aliasing filter before an ADC.
The circuit shown in Figure 1 uses a single MAX4450 op-amp. The insertion loss is >20db at 13.5MHz,
and >40db at 27MHz.
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