LT1229 Linear Technology, LT1229 Datasheet - Page 10

• Manufacturer Part Number: LT1229
• Description: Dual and Quad 100MHz Current Feedback Amplifiers
• Manufacturer: Linear Technology

LT1229/LT1230
A
Crosstalk and Cascaded Amplifiers
The amplifiers in the LT1229/LT1230 do not share any
common circuitry. The only thing the amplifiers share is
the supplies. As a result, the crosstalk between amplifiers
is very low. In a good breadboard or with a good PC board
layout the crosstalk from the output of one amplifier to the
input of another will be over 100dB down, up to 100kHz
and 65dB down at 10MHz. The following curve shows
the crosstalk from the output of one amplifier to the
input of another.
10
TYPICAL
Single 5V Supply Cable Driver for Composite Video
This circuit amplifies standard 1V peak composite video
input (1.4V
terminated cable. In order for the output to swing
2.8V
rately. The average DC level of the composite input is a
function of the luminance signal. This will cause problems
if we AC couple the input signal into the amplifier because
a rapid change in luminance will drive the output into the
rails. To prevent this we must establish the DC level at the
input and operate the amplifier with DC gain.
The transistor’s base is biased by R1 and R2 at 2V. The
emitter of the transistor clamps the noninverting input of
the amplifier to 1.4V at the most negative part of the input
PPLICATI
P-P
on a single 5V supply, it must be biased accu-
120
110
100
P-P
90
80
70
60
50
Amplifier Crosstalk vs Frequency
10
A
) by two and drives an AC coupled, doubly
O
PPLICATI
100
U
S
1k
FREQUENCY (Hz)
I FOR ATIO
10k
U
100k
O
U
V
A
R
R
1M
S
V
S
L
W
= ±15V
= 10
= 50
= 100
S
LT1229 • TA12
10M
100M
U
The high frequency crosstalk between amplifiers is
caused by magnetic coupling between the internal wire
bonds that connect the IC chip to the package lead frame.
The amount of crosstalk is inversely proportional to the
load resistor the amplifier is driving, with no load (just
the feedback resistor) the crosstalk improves 18dB. The
curve shows the crosstalk of the LT1229 amplifier B
output (pin 7) to the input of amplifier A. The crosstalk
from amplifier A’s output (pin 1) to amplifier B is about
10dB better. The crosstalk between all of the LT1230
amplifiers is as shown. The LT1230 amplifiers that are
separated by the supplies are a few dB better.
When cascading amplifiers the crosstalk will limit the
amount of high frequency gain that is available because
the crosstalk signal is out of phase with the input signal.
This will often show up as unusual frequency response.
For example: cascading the two amplifiers in the LT1229,
each set up with 20dB of gain and a –3dB bandwidth of
65MHz into 100
response will start to drop at about 10MHz and then flatten
out from 20MHz to 30MHz at about 0.5dB down. This is
due to the crosstalk back to the input of the first amplifier.
For best results when cascading amplifiers use the LT1229
and drive amplifier B and follow it with amplifier A.
(the sync pulses). R4, R5 and R6 set the amplifier up with
a gain of two and bias the output so the bottom of the sync
pulses are at 1.1V. The maximum input then drives the
output to 3.9V.
V
IN
5V
1 F
2N3904
C1
+
R3
150k
R1
3k
R2
2k
750
R5
will result in 40dB of gain, BUT the
C2
1 F
R4
1.5k
+
–
LT1229
1/2
R6
510
1000 F
+
C3
47 F
C4
75
R7
LT1229 • TA11
10k
R8
V
OUT