DN343 - Video Difference Amplifier Brings Versatility to Low Voltage Applications

Video Difference Amplifier Brings Versatility
to Low Voltage Applications – Design Note 343
Jon Munson
Introduction
The LT®6552 is a specialized dual-differencing 75MHz
operational amplifier ideal for rejecting common mode
noise as a video line receiver. The input pairs are designed to operate with equal but opposite large-signal
differences and provide exceptional high frequency common mode rejection (CMRR of 65dB at 10MHz), thereby
forming an extremely versatile gain block structure that
minimizes component count in most situations. The
dual input pairs are free to take on independent common mode levels, while the two voltage differentials
are summed internally to form a net input signal. The
LT6552 is optimized for low supply voltages, ranging
from 3V to 12V between the rails. In single supply
operation at 3.3V, the input range includes ground and
an output that can swing within 400mV of either supply
rail when driving a 150Ω load. Additionally, a shutdown
feature is provided to allow easy power management
in supply current-sensitive applications.
Dual Input Pair Zaps Common Mode Noise Pickup
The basic application of the LT6552 is that of a high
CMRR difference amplifier as shown in Figure 1. Only
two resistors are required to set the difference gain
(Gain = 2 in this example). For unity gain, no additional
components are required. Single supply operation
usually requires a means of output offset control so
that signals aren’t clipped. The dual input structure of
the LT6552 makes this easy (see VDC(ADJ) in Figure 1).
The circuit shown in Figure 1 is especially effective in
removing ground noise from video signals in vehicular
and industrial applications, as shown in Figure 2, killing common mode noise by over a factor of 1000 and
providing a lower solution cost than other op amp based
topologies because of the fewer required components.
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VIN
5V
3
2
CABLE
VDC(ADJ)
1
8
+
–
REF
FB
7
LT6552
6
75Ω
VOUT
4
75Ω
RF
500Ω
RG
500Ω
RECOVERED
OUTPUT
DN343 F01
CF
8pF
Figure 1. Noise Rejecting Sense Amplifier with DC Adjust
08/04/343_conv
NOISY
INPUT
DN343 F02
Figure 2. Common Mode Noise Eliminated
from Video Signal
Perform Video Rate Analog Arithmetic
Because of its dual differencing input structure, the
LT6552 is able to readily process both additive (noninverting) and subtractive (inverting) variables without the
need for complicated resistor networks. Figure 3 shows
that the output is a function of three input variables with
SHDN
V3
V2
V1
3
2
1
8
5
+
–
V+
7
REF
FB
6
LT6552
VO
Conclusions
By virtue of its dual differencing input structure and
ability to operate from low supply voltages, the LT6552
provides a versatile and high performing gain block
for modern analog applications. Of particular value is
the part’s ability to recover difference signals in the
presence of strong common mode interference, such
as in vehicular and closed-circuit video applications.
4
RF
RG
VO = (V1 – V2 + V3)
DN343 F03
RF + RG
RG
Figure 3. General Purpose Arithmetic Block
3V
499Ω
499Ω
the overall scaling set by the feedback resistors. This
property provides a useful means of differential to single
ended conversion with offset control or performing
multivariate functions such as the YPBPR to RGB video
adapter application shown in Figure 4. The circuit of
Figure 4 uses the fewest possible amplification stages
to accomplish the needed matrix conversion functions
while operating on the lowest supply voltages (±3V).
Input black levels should be near 0V for best results
and sync on Y is mapped to all three outputs, though
typically only needed on G.
3V
499Ω
8
8
7 8.2pF
FB
1
REF
6
LT6552
2
– SHDN
3
+
5
4
1
2
3
7
FB
REF
–
+
LT6552
SHDN
4
499Ω
5.6pF
6
75Ω
G
75Ω
5
–3V
–3V
3V
499Ω
909Ω
Y
8
1
1R
21.5Ω
21.5Ω
53.6Ω
11.3Ω
2
3
FB
REF
7
LT6552
2.2pF
6
75Ω
R
– SHDN
+
5
75Ω
4
–3V
42.2Ω
3V
499Ω
8
1
1B
2
49.9Ω
25.5Ω
3
3:t1R
(:ot1Bot1R
#:t1B
FB
REF
–
+
7
LT6552
SHDN
4
–3V
1.30k
1pF
6
75Ω
B
75Ω
5
BW (±0.5dB) > 25MHz
BW (–3dB) > 36MHz
IS ≈ 70mA
DN343 F04
Figure 4. YPBPR to RGB Component Video Converter
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn343f_conv LT/TP 0804 305K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 2004
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