OPA2830 Datasheet, PDF (24/43 Pages) National Semiconductor (TI) – Dual, Low-Power, Single-Supply, Wideband OPERATIONAL AMPLIFIER

OPA2830
SBOS309D – AUGUST 2004 – REVISED AUGUST 2008.................................................................................................................................................. www.ti.com
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 2 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
gives about a 200mV input DC offset that will show
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 acts to hold the
output in its linear operating region. This will pass 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 75
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
10 10² 10³ 10⁴ 10⁵ 10⁶ 10⁷ 10⁸ 10⁹
Frequency (Hz)
Figure 75. Video Line Driver Response to Matched
Load
+5V
Video DAC
1.87kΩ
78.7Ω
1/2
OPA2830
325Ω
650Ω
47µ F
845Ω
528Ω
75Ω
22µF
V_O
75Ω Load
Figure 76. Video Line Driver with SAG Correction
24
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