TSH343

Application information

11/17

Figure 27.

Detailed view of one TSH343 channel

Because of the shape of the signal shown in Figure 25, we use a very low output rail triple

high-speed buffer. The TSH343 supplied in 5V single power supply, features a low output rail

of 40mV on 150-ohm load. The TSH343 is used to drive high definition video signals up to

30MHz on 75-ohm video lines. It is dedicated to driving YPbPr signals where the

synchronization tip—close to zero volt—is included in the Y signal.

Figure 27 shows a solution used on the STMicroelectronics reference design of STi7100 or

STi7200 where the DAC output is loaded by 140

Ω and the bottom of the synchronization tip

is set at 10mV. Using the TSH343, an internal input DC value of 600mV is added to the

video signal in order to shift the bottom from 10mV to 610mV. The shift is not based on the

average of the signal, but is an analog summation of a DC component to the video signal.

Therefore, no input capacitors are required which provides a real advantage in terms of cost

and board space.

The internal gain of 2 obtained makes it possible to remove two resistors on the BOM. To

avoid any perturbation on matching from the DACs output impedance along a large band of

30MHz in HD, a discrete reconstruction filtering is implemented after the driver. This filter is

matched on 75-ohms. Note that the TSH343 cannot be AC output coupled (it cannot sink an

output current, therefore it is not possible to implement an output series capacitor).

video line

+5V

1/3 TSH343

(gain=2)

0V

0Volt

5Volt

DAC

68pF

68pF

470nH

75

Ω

75

Ω

TV

STB

1,22V

(10mV+600mV)*gain

3,22V

0Volt

5Volt

10mV

1,01V

0Volt

610mV

1,61V

140

Ω

+

600mV

DC

-5V

5Volt