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ADV7188 - Problems with White peak algorithm

Question asked by Joaquin on Mar 23, 2012
Latest reply on Mar 30, 2012 by DaveD

Hello:

 

We are writing the control program of an audio and video digitizer which uses the ADV7188. This IC has an automatic gain control based on a "peak white" algorithm (activated with LAGC=010).

 

When we raise the level of the input signal, the circuit does as it is supposed to do, keeping the output level constant.

But when we slowly lower the input signal level, the output level is also slowly reduced. It doesn't keep the output level constant, and we are troubled by this assymetric behaviour.

 

As the datasheet says, "Analog Devices internal parameters can be used to customize the peak white gain control. Contact an Analog Devices representative for more information.", but our region's representative forwarded us to this message board.

 

Some puntualizations:

  • We use the 17th line with a white reference level, so there is no need to use the historic maximum white level.
  • Actually, as we slowly lower the input level, the output also goes slowly down, but when it reaches a minimum level it suddenly goes back to a higher level. Lowering even further the input level reduces the output level from here (from this high output level). As we lower the input signal level down to zero, this cycle repeats several times. To explain it in other words, the gain of the IC does something as a staircase when lowering the input level, but it has no steps when rising the input level.

 

If possible, we would want to know the way the "peak white" algorithm works, and how to tune its behaviour to have no steps in the gain when lowering the input level. We suppose its controls are part of the "hidden space", out of the normal register maps, (accessed by writing a 0x80 on the register 0x0E), and there is no information about it on the datasheet.

 

Thanks in advance.

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