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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):

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