Hello
The ADC in LPC2148 operates at 4.5MHz or less according to the user manual(UM10139) provided. But from what I experienced, the ADC gives the output upto 15MHz.
I set the System clock at 60MHz using PLL. Also in the ADC control register using the CLKDIV bits I have to divide the system clock with the appropriate value such that the frequency becomes less than or equal to 4.5 MHz. But even if it is more than 4.5MHz, it gives output till I set the clock to 15MHz.
Am I missing something that I need to know here or the LPC2148 works this way?
Yes.
A specification - such as the UM10139 - tells you under what conditions a thing is guaranteed to work. It does not mean that the thing will just immediately stop working completely the moment you exceed any one parameter - just that the performance and/or behaviour becomes undefined.
This has absolutely nothing to do with the LPC2148 specifically, or with Keil - it applies entirely in general.
There is also, of course, the possibility that your configuration is not doing as you think ...