TA0310 View Datasheet(PDF) - STMicroelectronics
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TA0310 Datasheet PDF : 14 Pages
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TA0310
8 Impedance matching
Figure 7: Typical connexion between set-top-box and TV
Impedance matching
SET-TOP-BOX
1Vpp 2Vpp
+
Gain=2 -
75Ω
0Volt
R
R
0Volt
video line
TV
75Ω
1Vpp
0Volt
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 75Ω 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 75Ω is physically implemented on the board to achieve the right value for matching. A second resistor
of 75Ω (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. 1kΩ 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=680Ω 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.
1 Reflection factor occurs when the line is loaded by the same value as its own characteristic impedance Zc.
6/14
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