Video Limiter

To avoid exceeding luminance reference level standard or to avoid exceeding the input range of digitizer (ADC), video signal is often needed to be limited. The simple way to do this is by hard limiting the signal in the positive direction (white peak clipping), but this method completely destroy all information contained in the clipped region.

The better way to limit the signal while preserving all information contained in the signal is by soft limiting the signal, where the signal will be compressed at the above threshold region. Here is the schematic diagram of the circuit:

The LT1228 is used here in a slightly unusual, closed-loop configuration. The gain of the closed-loop is set by the feedback and gain resistors (RF and RG) and the open-loop gain by the transconductance of the first stage times the gain of the CFA. The level at which the limiting action begins is adjusted by varying the set  current into pin 5 of the transconductance amplifier. [Schematic diagram source: Linear Technology Application Notes]