Datasheet

Video Drive Circuits
SINGLE SUPPLY OPERATION
+
−
Video In
10 µF
0.1 µF
V
S−
75 Ω
75 Ω
10 µF 0.1 µF
1.43 kΩ1.43 kΩ
75 Ω
V
S+
+
5
3
4
2
+
1
V
O
_
+
49.9 Ω
50 Ω Source
V
I
+V
S
V
O
R
f
1.3 kΩ
R
g
1.3 kΩ
+V
S
2
+V
S
2
_
+
1.3 kΩ
50 Ω Source
V
I
V
S
V
O
R
f
1.3 kΩ
52.3 Ω
R
g
499 Ω
R
T
499 Ω
R
T
+V
S
2
+V
S
2
+
−
Video In
10 µF
0.1 µF
V
S−
75 Ω
75 Ω
10 µF
0.1 µF
1.43 kΩ1.43 kΩ
75 Ω
V
S+
+
5
3
4
2
+
1
75 Ω
75 Ω
75 Ω
75 Ω
V
O
V
O
V
O
SN10501
SN10502
SN10503
www.ti.com .................................................................................................................................................. SLOS408B – MARCH 2003 – REVISED JANUARY 2009
as the noninverting circuit in Figure 33 . However, the
amplifier output now sees the 100- Ω feedback
Most video-distribution systems are designed with
resistor in parallel with the external load. To eliminate
75- Ω series resistors to drive a matched 75- Ω cable.
this excessive loading, increase both R
g
and R
f
,
In order to deliver a net gain of 1 to the 75- Ω
values, as shown in Figure 34 , and then provide the
matched load, the amplifier is typically set up for a
input-matching impedance with a third resistor (R
M
) to
voltage gain of 2, compensating for the 6-dB
ground. The total input impedance becomes the
attenuation of the voltage divider formed by the series
parallel combination of R
g
and R
M
.
and shunt 75- Ω resistors at either end of the cable.
The last major consideration to discuss in inverting The circuit shown in Figure 36 meets this
amplifier design is setting the bias-current requirement. The SN1050x gain flatness and
cancellation resistor on the noninverting input. If the differential gain/phase performance provide
resistance is set equal to the total dc resistance exceptional results in video distribution applications.
looking out of the inverting terminal, the output dc
error, due to the input bias currents, is reduced to the
input-offset current multiplied by R
f
in Figure 34 . The
dc source impedance looking out of the inverting
terminal is 1.3 k Ω || (1.3 k Ω + 25.6 Ω ) = 649 Ω . To
reduce the additional high-frequency noise introduced
by the resistor at the noninverting input, and
power-supply feedback, R
T
is bypassed with a
capacitor to ground.
The SN1050x family is designed to operate from a
single 3-V to 15-V power supply. When operating
from a single power supply, care must be taken to
ensure that the input signal and amplifier are biased
appropriately to allow for the maximum output voltage Figure 36. Cable Drive Application
swing. The circuits shown in Figure 35 demonstrate
methods to configure an amplifier for single-supply
Differential gain and phase measure the change in
operation.
overall small-signal gain and phase for the color
subcarrier frequency (3.58 MHz in NTSC systems) vs
changes in the large-signal output level (which
represents luminance information in a composite
video signal). The SN1050x, with the typical 150- Ω
load of a single matched video cable, shows less
than 0.007% / 0.007 ° differential gain/phase errors
over the standard luminance range for a positive
video (negative sync) signal.
Figure 35. DC-Coupled Single Supply Operation
Figure 37. Video Distribution
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Product Folder Link(s): SN10501 SN10502 SN10503