THS7353 Datasheet, PDF (30/51 Pages) Texas Instruments – 3-Channel Low Power Video Buffer with I2C Control, Selectable Filters, External Gain Control, 2:1 Input MUX, and Selectable Input Modes

THS7353
SLOS484 – NOVEMBER 2005
www.ti.com
APPLICATION INFORMATION (continued)
A benefit of the externally configured gain control is it allows for frequency gain manipulation. There are two main
reasons to do this.
• The first reason is to account for skin-effect losses in cables. Skin-effect accounts for the high-frequency
current flow at the edges of a conductor. This results in an increase of resistance as frequency increases.
This high frequency resistance is proportional to the square-root of the frequency.
• For very short cable lengths, the skin effect can be generally ignored - especially for SD frequencies. But, if
the cable length is greater than 10-meters for G'B'R' or HD signals, this effect can start to cause losses in
high frequency signal amplitude. This would generally appear as a loss in sharpness on a video monitor.
One way to counter-act, or equalize, the skin-effect loss is to increase the gain of the amplifier at the same rate
of attenuation. The difficult problem with equalization is that skin effect is a function of the square-root of the
frequency. Hence, adding a simple RC zero network does not accurately equalize the loss. But, if combinations
of RC zeroes spread throughout the frequency spectrum are used, such as the one shown in Figure 66, then a
close approximation to skin-effect losses can be equalized out of the system. The amount of equalization is
dependant on the length of the cable and the type of cable used. So, fixing the equalization to account for a long
length of cable causes significant peaking issues when a short cable is used.
Input A
Input B
0.1 mF
In A
In B
75 W
0.1 mF
75 W
1 of 3 Channels
Bypass
2:1
X1
DC +
250 mV
DC
AC -
BIAS
AC
Sync
TIP
Clamp
MUTE
LPF
9 / 16 /
35 MHz
I2C-
SDA
I2C-
SCL
0.1 mF
+
Out
-
1 kW
Gain
Adjust
250 W
750 W
3.3 V
15 pF
3.3 V
ADC /
Video
Decoder
10 pF
Cable
Equalization
Circuit
Figure 66. Skin-Effect Loss Compensation (Equalization) Input Buffer Configuration Example
Similar to skin-effect compensation, the other advantage of having control of the amplifier gain is for Sin-X/X
compensation. DACs have a roll-off at high-frequencies approximating a Sin-X/X loss. This is dependant on the
DAC, the sampling frequency, and the desired frequency of interest. Due to the numerous numbers of DACs and
video encoders available, the Sin-X/X compensation must be adjusted depending on the system. An example of
a dc-coupled Sin-X/X compensation circuit with a 6-dB gain is shown in Figure 67.
3.3 V
DAC /
Encoder
DC + 250 mV
R
R
1 of 3 Channels
Bypass
2:1
DC +
250 mV
DC
AC -
BIAS
X1
+
LPF
9 / 16 /
-
AC
35 MHz
Sync
TIP
Clamp _MUTE
I2C-SDA I2C-SCL
1 kW
250 W
3.3 V
Out
2.2 pF
750 W
75 W
Video
Out
75 W
1 kW
2.2 pF
Sin-X/X
Compensation
With 6-dB Gain
Figure 67. Sin-X/X Compensation with 6-dB Gain Output Buffer Configuration Example
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