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LT1251

40MHz Video Fader and DC Gain Controlled Amplifier

Linear Technology 

LT1251/LT1256

APPLICATIONS INFORMATION

100

VFS = 2.5V

millivolts of the negative supply can drive the NULL pin.

The AM modulator application shows an LT1077 driving

the NULL pin to eliminate the output DC offset voltage.

RF2 = 4.3k

50

RF2 = 1.5k

0

0

0.5 1.0 1.5 2.0 2.5

CONTROL VOLTAGE (V)

1251/56 F01

Figure 1. Linear Gain Control from 0 to 101

Capacitive Loads

Increasing the value of the feedback resistor reduces the

bandwidth and open-loop gain of the LT1251/LT1256;

therefore, the pole introduced by a capacitive load can be

overcome. If there is little or no resistive load in parallel

with the load capacitance, the output stage will resonate,

peak and possibly oscillate. With a resistive load of 150Ω,

any capacitive load can be accommodated by increasing

the feedback resistor. If the capacitive load cannot be

paralleled with a DC load of 150Ω, a network of 200pF in

series with 100Ω should be placed from the output to

ground. Then the feedback resistor should be selected for

best response.

The Null Pin

Pin 6 can be used to adjust the gain of an internal current

mirror to change the output offset. The open circuit

voltage at Pin 6 is set by the full scale current IFS flowing

through 200Ω to the negative supply. Therefore, the NULL

pin sits 100mV above the negative supply with VFS equal

to 2.5V. Any op amp whose output swings within a few

Crosstalk

The amount of signal from the off input that appears at the

output is a function of frequency and the circuit topology.

The nature of a current feedback input stage is to force the

voltage at the inverting input to be equal to the voltage at

the noninverting input. This is independent of feedback

and forced by a buffer amplifier between the inputs. When

the LT1251/LT1256 are operating noninverting, the off

input signal is present at the inverting input. Since one end

of the feedback resistor is connected to this input, the off

signal is only a feedback resistor away from the output.

The amount of unwanted signal at the output is deter-

mined by the size of the feedback resistor and the output

impedance of the LT1251/LT1256. The output impedance

rises with increasing frequency resulting in more crosstalk

at higher frequencies. Additionally, the current that flows

in the inverting input is diverted to the supplies within the

chip and some of this signal will also show up at the

output. With a 1.5k feedback resistor, the crosstalk is

down about 86dB at low frequencies and rises to – 78dB

at 1MHz and on to – 60dB at 6MHz. The curves show the

details.

Distortion

When only one input is contributing to the output (VC = 0%

or 100%) the LT1251/LT1256 have very low distortion. As

the control reduces the output, the distortion will increase.

The amount of increase is a function of the current that

flows in the inverting input. Larger input signals generate

more distortion. Using a larger feedback resistor will

reduce the distortion at the expense of higher output

noise.

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