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6 / 14 page 6/14 TA0310 Impedance matching 8 Impedance matching We can summarize as follows the constraints met when driving a signal on a line (these are constraints that can be found in any textbook on the theory of line transmission): In order to remove any reflection factors(1), the line must be loaded on both sides by its own characteristic impedance; typically 75 Ω for video lines. We call this impedance matching because the impedance is equivalent at any point in a given line. As the output impedance of the op-amp is close to zero, a resistor of 75 Ω is physically implemented on the board to achieve the right value for matching. A second resistor of 75 Ω (TV side) allows matching on the other side. As show in Figure 7, the network behaves like a resistor divider for the signal amplitude. Because of this, half of the output amplitude of the op-amp is lost. As the input amplitude of the op-amp must be the same as the amplitude required on the line (typically 1Vpp in video), a gain of +2 (6dB) is required on the op- amp. The value of R must be as small as possible to reduce noise and the problems of stability (assuming stray capacitances mainly on inverting input), but not too small as the 2R network is viewed as a load by the op- amp output. For a VFA, the value of R is not imposed. 1k Ω is a good choice and it satisfies the previous requirements. For a CFA, as TSH11x, the value of R is imposed and it is available in the datasheet (R=680 Ω for gain=+2). 9 Power supply A constraint belonging to every designer is the need to reduce the cost of his application. A dual power supply -5V/+5V requires an investment in a negative -5V supply circuit. One solution is to reduce the power supply to a single supply 0/+5V. As described in Figure 6, the synchronization signal descends to 0V (sometimes only 10mV). In cases such as these, the best solution is to use an input/output rail-to-rail op-amp such as TSH7x-TSH8x families. Assuming the tested value of the output rail is VOL=150mV max. Figure 7: Typical connexion between set-top-box and TV 1 Reflection factor occurs when the line is loaded by the same value as its own characteristic impedance Zc. video line 75 Ω 75 Ω R 0Volt 0Volt 2Vpp 1Vpp 0Volt 1Vpp R + - SET-TOP-BOX TV Gain=2 |
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