on the feedback type of AGC,How to prevent the delay of the delay of the operation?
in sigma delta modulation,decimation and envelope detector will cause some delay.
How to prevent it for AGC?
If you are looking at digital implementation you can easily achieve <1ms time constant if your ADC is not too complex. However except in case of high speed mobile communication you do not require fast AGCs. So do check what is the time constant you require. If it is very low you need a fast implementation.
Do look at some specs of the GPS RF Frontends they give you a lot of ideas about these numbers. It may not be suitable for your application but will be good.
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