THS7303 Datasheet, PDF (29/43 Pages) Texas Instruments – 3-Channel Low Power Video Amplifier with I2C Control, Selectable Filters, 6-dB Gain, SAG Correction, 2:1 Input MUX, and Selectable Input Bias Modes

THS7303
www.ti.com
SLOS479 – OCTOBER 2005
APPLICATION INFORMATION (continued)
OUTPUT MODES OF OPERATION – AC COUPLED WITH SAG CORRECTION
Other than the line droop issue, ac coupling has another potential issue – size and cost. A 330-µF to 1000-µF
capacitor is large and can be quite costly in a system. Multiply these items by the number of channels, and the
size and costs can be significant. But, it is still desirable to use ac coupling to eliminate ground loop issues and
insure interoperability among video devices.
The SAG nomenclature represents signal amplitude gain correction in this document. SAG correction is a
method which is used to ac couple the video signal while using much smaller value capacitors. SAG correction is
accomplished by manipulating the feedback network of the output buffer. The THS7303 was designed to take
advantage of this compensation scheme while minimizing the number of external components required.
Figure 66 shows the basic configuration of the output buffer stage along with the SAG configuration driving a
single video line.
Signal
1 kW
878 W
Internal
Circuitry
Out ^{47 mF}
675 W
33 mF
SAG
150 W
75 W
Video
Out
75 W
Figure 66. THS7303 Output Buffer Using SAG Corrected AC-Coupling
SAG compensation can be analyzed by looking at low frequency operation and high frequency operation. At low
frequencies, the impedance of the capacitors are high and the corresponding gain of the amplifier is:
ǒ Ǔ 1 )
(675 ) 878)
1k
+ 2.55 VńV () 8.1 dB).
(1)
But, at high frequencies, the impedance of the capacitors are low and the resulting gain of the amplifier is:
ǒ Ǔ ǒ Ǔ 1 )
ƪ_{(675 ø 150) ) 878}ƫ
1k
+1)
1k
1k
+ 2 VńV () 6.0 dB)
(2)
which is needed to counter-act the doubly terminated 75-Ω output divider (-6 dB) circuit. Resulting in the video
out signal equaling the Input signal amplitude.
When the SAG output pin is connected directly to the amplifier output, as found in the dc-coupled and the
ac-coupled configurations, the gain is configured properly at 2 V/V (6 dB). The SAG pin is part of the negative
feedback network. Thus, the capacitors and traces should be constructed as close as possible to the THS7303 to
minimize parasitic issues. Failure to do so may result in ringing of the video signal.
If these large capacitors must be placed further than 15 mm away from the THS7303, it is recommended that a
0.01-µF capacitor be placed between the output of the channel and the SAG pin. This capacitor should be placed
as close as possible to the THS7303 to minimize stray capacitance and inductance issues. Since SAG correction
targets the low frequency operation area, there is no drawback of adding this high frequency capacitor to the
circuit.
When SAG correction is used, low frequency gain is higher than the high frequency gain (8.1 dB vs. 6 dB). This
gain counter acts the attenuation of the signal due to the increase in the 47-µF capacitor impedance. This
amplifier gain increase is determined by the 33-µF capacitor (and associated internal resistor values) and causes
a Q enhancement to occur at low frequencies – typically at about 15-Hz. The ratio of these capacitors
determines the frequency and amplitude of this enhancement.
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