OPA4830 Datasheet, PDF (24/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

OPA4830
SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com
+5V
Video DAC
1.87kW
78.7W
1/4
OPA4830
325W
650W
47mF
845W
528W
75W
22mF
V_O
75W Load
Figure 76. Video Line Driver with SAG Correction
AC-COUPLED OUTPUT VIDEO LINE DRIVER
Low-power and low-cost video line drivers often
buffer digital-to-analog converter (DAC) outputs with
a gain of 2V/V into a doubly-terminated line. Those
interfaces typically require a dc blocking capacitor.
For a simple solution, that interface often has used a
very large value blocking capacitor (220µF) to limit
tilt, or SAG, across the frames. One approach to
creating a very low high-pass pole location using
much lower capacitor values is shown in Figure 76.
This circuit gives a voltage gain of 2 at the output pin
with a high-pass pole at 8Hz. Given the 150Ω load, a
simple blocking capacitor approach would require a
133µF value. The two much lower valued capacitors
give this same low-pass pole using this simple SAG
correction circuit of Figure 76.
The input is shifted slightly positive in Figure 76 using
the voltage divider from the positive supply. This
configuration gives about a 200mV input dc offset
that shows up at the output pin as a 400mV dc offset
when the DAC output is at zero current during the
sync tip portion of the video signal. This offset acts to
hold the output in its linear operating region. This
circuit then passes on any power-supply noise to the
output with a gain of approximately –20dB, so good
supply decoupling is recommended on the
power-supply pin. Figure 77 shows the frequency
response for the circuit of Figure 76. This plot shows
the 8Hz low-frequency high-pass pole and a high-end
cutoff at approximately 100MHz.
3
0
-3
-6
-9
-12
-15
-18
-21
1
2
3
4
5
6
7
8
9
10 10 10 10 10 10 10 10 10
Frequency (Hz)
Figure 77. Video Line Driver Response to Matched
Load
24
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