| Video signal level conversion of DC coupled video amplifier / filter |
| Abstract: This application note describes a method for level-shifting video signals from -0.3V to 0.7V to 0 to 1V. Maxim provides chips for filtering and amplifying analog video signals. Most of these chips are connected to the output of a video digital-to-analog converter (DAC). In order to understand the video characteristics of these video amplifiers / filters, standard test signals (such as color bar signals) are used for testing. Most video test templates have a video test signal range of -0.3V to 0.7V, but most of Maxim's chips require a video signal range of 0 to 1V. The circuit shown in Figure 1 can convert the output level of the video test signal generator to 0 to 1V. This circuit contains a first-stage amplifier with a gain of -1, followed by a second-stage amplifier with a gain of -2. Therefore, the overall gain is +2. The output DC level can be adjusted by adjusting the sliding end of the potentiometer at the non-inverting input of the second-stage amplifier. The input resistance of the level shift circuit shown in Figure 1 is about 75Ω. The 75Ω resistance is obtained by connecting a 100Ω resistance to ground in parallel with the 300Ω input resistance of the inverting amplifier.  Figure 1. Level shift circuit Figure 2 shows the standard NTSC composite video test signal (the color bar signal is generated by the Tektronix 1910 digital signal generator), and a DC offset is added to the signal.  Figure 2. Composite color bar video test signal Figure 3 shows a composite S video signal containing brightness and chroma (color bars are generated by Quantum Data 802 BT video test signal generator), and an offset voltage is added to the test signal for level conversion.  Figure 3. S-video color bar video test signal Figure 4 shows a 1080i format video test signal (generated by Quantum Data 802 BT video test signal generator), and an offset voltage is added to the test signal for level conversion.  Figure 4. 1080i format video test signal |
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