TSH80-TSH81-TSH82

Precautions on Asymmetrical Supply Operation

19/23

This example shows a possible application of the TSH8X circuit. Here, you can multiplex the

channels for the different standard PAL, NTSC as you filter for the different bands; the video

signal can be filtered with two different cutoff frequencies, corresponding to a PAL encoded

signal (LPF1) or a NTSC signal (LPF2).

You can multiplex input signals, as the outputs are in high impedance state in standby mode.

This enables you, to use a PAL filter as the Standby mode is active and to use the NTSC filter

otherwise.

The video application requires 1Vpeak at input and output.

Calculation of components:

A decoupling capacitor is provided to cutoff the frequencies below 10Hz according I bias.

Hence Ce=10uF, with Rb1=10k

Ω. At the output, Cout=220uF.

The AOP1 is in 6dB configuration for the adaptation bridge. R1=R2=1k

Ω,V1=2Vpk, V2=1Vpk

For the PAL communication, we need a low pass filtering. The load resistance R4 is function of

the output resistance of the filter.V3=V2/A1 where A1 is the attenuation factor of the filter LPF1.

To compensate the filter insertion loss, we add an additional factor to the gain of the 2nd

amplifier AOP2.

For example, for an attenuation of 3dB, we choose R5=300

Ω and R6=1kΩ. We have V4=2Vpk

and Vout=1Vpk.

The calculation of the parameters R7, C7, R8, C8, R9, R10 will be exactly the same

Table 7.

Capacitance Cf on the phase margin of the circuit

Parameter

Cf (pF)

Vcc=±1.5V

Vcc=±2.5V

Vcc=±5V

Unit

Phase Margin

0

28

43

56

deg

f-3dB

40

39.3

38.3

MHz

Phase Margin

5.6

30

43

56

deg

f-3dB

40

39.3

38.3

MHz

Phase Margin

22

37

52

67

deg

f-3dB

37

34

32

MHz

Phase Margin

33

48

65

78

deg

f-3dB

33.7

30.7

27.6

MHz

Figure 34. Example of a video application

IN

Ce

Re

Rb1

Vcc/2

+

-

Vcc/2

+

-

+

-

Standby

Standby

R1

R2

R3

C3

R4

C4

Rb1

R5

R6

R9

R10

Rout

OUT

AOP1

AOP2

AOP3

V1

V2

V3

V4

A1

Vcc/2

Vcc/2

Cf

Cf

Cf

Cout

Vcc/2

RL

R7

C7

R8

C8

LPF2

A2

PAL

NTSC

LPF1

Rb1

Vcc/2