LT1229/LT1230

10

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.

Amplifier Crosstalk vs Frequency

FREQUENCY (Hz)

10

50

60

70

80

90

100

120

100

1k

10k

100M

LT1229 • TA12

100k

1M

10M

110

S

APPLICATI

I FOR ATIO

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

Ω will result in 40dB of gain, BUT the

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.

S

APPLICATI

TYPICAL

Single 5V Supply Cable Driver for Composite Video

This circuit amplifies standard 1V peak composite video

terminated cable. In order for the output to swing

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

(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.

LT1229 • TA11

–

+

1/2

LT1229

R3

150k

R2

2k

R5

750

Ω

C1

1

µF

R8

10k

R1

3k

C2

1

µF

R4

1.5k

+

2N3904

5V

C3

47

µF

R6

510

Ω

R7

75

Ω

C4

1000

µF