AN-42041

APPLICATION NOTE

© 1999 Fairchild Semiconductor Corporation

www.fairchildsemi.com

AN-42041 Rev. 1.0.1

2

Procedure

Do not turn the power supply on until all connections shown

in Figure 2 are completed.

1. Set the power supply to 0V. Connect the power supply to

the FMS6400 Demo Board.

NOTE:

Use the shortest possible cables (50Ω or 75Ω) for all the

following video connections.

S-Video input of the monitor.

of the monitor.

5. Adjust the input voltage to 5V. Verify that the test board

6. Apply the video test signal to the test board. A high-

quality image should appear on the monitor screen.

NOTE:

It may be necessary to connect the video test signal di-

rectly into Line C of the monitor to ensure the test

signal is present and of high quality.

7. Switch the monitor input to line A (Y and COUT).

8. Note the effect of the filter action on the video signal.

9. Switch the monitor input to Line B (CVOUT).

10. Note the effect of the filter action on the video signal.

Figure 2. Connection Diagram

Monitor

FMS6400 Test Board

+5V

GND

DC Power Supply

+

–

Y

Signal Source

IN

Line A

Results

The Bode plot in Figure 3 is a non-subjective measure of the

FMS6400 filtering action. A network analyzer was connected

to either input channel with a 200mV RMS test signal and a

plot was made of the frequency response of output vs. input.

The resulting amplitude vs. frequency plot demonstrates the

accuracy of the FMS6400 regarding flatness of response: 3dB

(cutoff) point at 7.1MHz and linear 50dB/decade rolloff

above cutoff.

Time domain measurements are shown in Figures 4 and 5.

Note the filtering action performed on the ‘choppy’ DAC

input signal. The video signal was a standard color-bar pattern.

The luma channel (Figure 4) contains the video signal ampli-

tude, while the chroma channel (Figure 5) carries the color

information. Note that the D/A artifacts are reduced by at

least a factor of 10. Though not shown, the composite output

is the filtered arithmetic sum of the luma and chroma chan-

nels.

Figure 6 illustrates applications where the video signal is

undersampled or requires a steeper rolloff. Two filters are

cascaded for a 160dB/decade rolloff. Note the use of 150Ω

termination resistors on U1’s output (Figure 7). This is done

to reduce the loading on U1 and preserve the DC restoration

capability of U2. Do not increase the termination resistor val-

ues above 240Ω. If the resistor values are increased,

decrease the series capacitors proportionately. This cascading

technique can be used with additional filters to obtain even

steeper rolloffs with minimal effect on the -3dB point.