40MHz Video Fader and DC Gain Controlled Amplifier

The LT1251/LT1256 are 2-input, 1-output, 40MHz current feedback amplifiers with a linear control circuit that sets the amount each input contributes to the output. These parts make excellent electronically controlled variable gain amplifiers, filters, mixers and faders. The only external components required are the power supply bypass capacitors and the feedback resistors. Both parts operate on supplies from ±2.5V (or single 5V) to ±15V (or single 30V).

Absolute gain accuracy is trimmed at wafer sort to minimize part-to-part variations. The circuit is completely temperature compensated.

The LT1251 includes circuitry that eliminates the need for accurate control signals around zero and full scale. For control signals of less than 2% or greater than 98%, the LT1251 sets one input completely off and the other completely on. This is ideal for fader applications because it eliminates off-channel feedthrough due to offset or gain errors in the control signals.

The LT1256 does not have this on/off feature and operates linearly over the complete control range. The LT1256 is recommended for applications requiring more than 20dB of linear control range.

Applications

* Composite Video Gain Control

* RGB, YUV Video Gain Control

* Video Faders, Keyers

* Gamma Correction Amplifiers

* Audio Gain Control, Faders

* Multipliers, Modulators

* Electronically Tunable Filters

* Accurate Linear Gain Control: ±1% Typ, ±3% Max

* Constant Gain with Temperature

* Wide Bandwidth: 40MHz

* High Slew Rate: 300V/µs

* Fast Control Path: 10MHz

* Low Control Feedthrough: 2.5mV

* High Output Current: 40mA

* Low Output Noise

* 45nV/√Hz at AV = 1

* 270nV/√Hz at AV = 100

* Low Distortion: 0.01%

* Wide Supply Range: ±2.5V to ±15V

* Low Supply Current: 13mA

* Low Differential Gain and Phase: 0.02%, 0.02°