AN1028| Application Note
Maxim > App Notes > FILTER CIRCUITS (ANALOG)
VIDEO CIRCUITS
Keywords: 3-pole, butterworth, active filter, low pass filter, video, output, NTSC, PAL, reconstruction filtering, operational amplifier, op amp, opamps
Mar 28, 2002
APPLICATION NOTE 1028
SDTV Reconstruction Filter with Adjustable Group Delay
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 75!&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 75!& 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.
Figure 1. A 5MHz, 3-Pole, Butterworth active filter--using a Sallen-Key realization--with adjustable group delay. To preserve the quality of the video waveform, it's important that the filter's group-delay variation, and the group-delay differential between filters, be as low as possible. Some means of adjusting group delay without
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