Why is the PGA always needed?

[machael]

hi, all:

I have a stupid question. why digital communication receiver always have a PGA(programable gain amplifier) in the receiving front-end? why not just adjust the amplification to the max? if the noise is bigger than the ADC receiving range, what will happen?

thank you!

 

[mowgli]

Re: Why PGA is needed?

The scope of the AGC block is to extend the dynamic range beyond the ADC input range: consider a 12 bit ADC with a theoretical dynamic range of about 72 dB. If the input signal increases beyond the maximum input level the ADC will saturate with consequent loss of information.
If you put an AGC block before the ADC you can control the amplifier gain such a way to keep the ADC input signal at a suitable level for the subsequent digital processing (e.g avoid saturation and keep 2 or 3 bit as margin for fast level variations not tracked by the VGA).
In this way you have extended the dynamic range of the system to the sum (theoretically) of ADC input dynamic range + VGA gain range.

Regards
Mowgli