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© Semiconductor Components Industries, LLC, 2010
March, 2010 − Rev. 1
1 Publication Order Number:
AND8457/D
AND8457
SAG Correction
Prepared by Xavier Bignalet, Krista Tollefsrud
ON Semiconductor
Video amplifiers are typically used to drive 75 W coaxial
cables through an AC coupling capacitor. Unfortunately, in
order to maintain the quality of the video signal this output
capacitor must be made extremely large. However, as
portable applications become more compact there are
greater size restrictions to meet. A solution is a method
called SAG correction, also known as SAG compensation,
which is used in AC coupled video applications to allow a
smaller output capacitor. A low cost solution is the
NCS2561, a single channel video driver that integrates a
built−in SAG correction circuit.
Typically a 220 mF capacitor is used to couple the output
of the video driver. In certain applications or to satisfy
customer specifications, one may need a higher capacitance
value, for example 470 mF and sometimes up to 1 mF, to
improve field tilt effects. However, there are two major
problems with using large output capacitors. First of all, they
are large in size, so they do not fit most portable applications
like cell phones or digital cameras. Secondly, they are quite
costly and minimizing expenditures is a key design
objective. A solution is to use SAG compensation, reducing
the size of the output coupling capacitor. Changing the
output capacitance affects the frequency response of the
high pass filter, composed of the output coupling capacitor
and the load resistor. With the introduction of the C
sag
capacitor a peaking begins to occur at lower frequencies. As
the value of the SAG capacitor is decreased the cutoff
frequency increases attenuating parts of the desired signal.
Therefore it is important to choose the lowest capacitance
values while still maintaining a low cutoff frequency. The
purpose of this application note is to determine SAG
capacitances that provide a comparable solution to standard
coupling without the use of large capacitors.
The goal of the SAG correction is to reduce the size of the
output capacitor without degrading the video performance.
The SAG correction offers a smaller and cheaper output
capacitor solution by splitting the original capacitor into two
smaller capacitors, which are referred to, in this text, as C
out
and C
sag
. When seeking the smallest values for these output
capacitors, the cutoff frequency is an important parameter
which should not be compromised.
Cutoff Frequency
Figure 1. Gain vs. Frequency Simulation 220 mF
Capacitor Configuration
1.0 3.0 10 30 100
−8.0
−4.0
0
4.0
8.0
FREQUENCY (Hz)
(4.8194,3.0048) −
C
out
= 220 mF − Csag disconnected
The cutoff frequency as shown below is dependent on the
line and load resistances, as well as the output capacitance.
f
C
+
1
2p
ǒ
R
Line
) R
Load
Ǔ
C
out
(eq. 1)
The 220 mF output capacitor produces a low −3 dB cutoff
frequency at approximately 5 Hz with the formula:
f
C
+
1
2p
ǒ
75 W ) 75 W
Ǔ
220 mF
[ 5Hz
(eq. 2)
A 470 mF capacitor will also provide a low cutoff frequency
around 2.3 Hz:
f
C
+
1
2p
ǒ
75 W ) 75 W
Ǔ
470 mF
[ 2.3 Hz
(eq. 3)
In the case where SAG correction is not used, as the value
of C
out
is decreased in a standard video circuit (Figure 3), the
cutoff frequency of the high pass filter approaches the
critical frequency bandwidth of 24 Hz − 30 Hz. This
bandwidth should not be attenuated because these are the
frequencies of the slower frame rate used by standard
definition television− NTCS (30 Hz), PAL & SECAM
(25 Hz), and ATSC (24 Hz).
http://onsemi.com
APPLICATION NOTE
Verzeichnis
- ・ Teilenummerierungssystem on Seite 7
- ・ Blockdiagramm on Seite 6
