APPLICATION NOTE 1799

5MHz, 3-Pole, Low-Pass Filter plus Video Line Driver for Consumer Video Applications


Abstract: The MAX4390 operational amplifier configured in a 3-pole, low-pass Sallen-Key filter that 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.

The Sallen-Key realization of a 5.25MHz, 3-Pole Butterworth filter shown in Figure 1 has a gain of 2V/V and is capable of driving 75Ω back-terminated coax to an overall gain of 1. Used to reconstruct component-video (Y, Pb, Pr) and RGB signals, this filter has an insertion loss of >20db at 13.5MHz and >40db at 27MHz (Figure 2). Like the anti-aliasing filter before an ADC, this filter is used to remove the higher-frequency replicas of a signal following a DAC.

Figure 1. This 3-pole Butterworth video-reconstruction filter has adjustable group delay.
Figure 1. This 3-pole Butterworth video-reconstruction filter has adjustable group delay.

Figure 2. Typical filter response for circuit of Figure 1 with R3 + R8 = 332ohm.
Figure 2. Typical filter response for circuit of Figure 1 with R3 + R8 = 332Ω.

To preserve quality in the video waveform, one should minimize group-delay variations in the filter and also any group-delay differential between filters. That capability requires a means for adjusting the filter's group delay without affecting its bandwidth. In Figure 1, the addition of R8 in series with C3 and R3 creates a lag-lead network. Keeping the sum of R3 and R8 constant and equal to the original R3 value preserves bandwidth by preserving the dominant-pole frequency. Increasing the R3 value, on the other hand, introduces a "lead" term that lowers group delay by reducing the rate of change in phase.

For R8 = 0Ω and R3 = 332Ω in the circuit shown, the average group-delay variation over the filter bandwidth is about 25ns Raising R8 to 31.6Ω and lowering R3 to 301Ω drops the variation to about 15ns, and setting R8 = 59Ω with R3 = 274Ω drops it to about 7ns The last case affects band-edge selectivity slightly (<0.5dB), but does not change the filter's -3dB bandwidth. These group-delay variations are shown in Figure 3.

Figure 3. Selected values of R8 and R3 (see text) allow control of group-delay variation over the filter's passband.
Figure 3. Selected values of R8 and R3 (see text) allow control of group-delay variation over the filter's passband.

A similar version of this article appeared in the August 8, 2002 issue of EDN magazine.


Related Parts
MAX4390 Free Sample
Next Steps
EE-Mail Subscribe to EE-Mail and receive automatic notice of new documents in your areas of interest.
Download Download, PDF Format
© , Maxim Integrated Products, Inc.
The content on this webpage is protected by copyright laws of the United States and of foreign countries. For requests to copy this content, contact us.
APP 1799:
APPLICATION NOTE 1799,AN1799, AN 1799, APP1799, Appnote1799, Appnote 1799