Hi Mustapha

> What I want to know if the pic16f628 has an in built-in  ADC

No, it doesn't. You might want to look at the F88 or one of the
other 18-pin micros

>http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=1034&mid=10&l
ang=en&pageId=74<

Perhaps even a skinny-DIP F877 ? You need 6 I/O lines
for the LCD (RS EN 4-bit data)

You should be able to sample a couple of what you want if
you can't but them locally

> and how the analog input works exactly ?

Any PIC datasheet and Mid-Range Reference Manual explain. Also
AN246, AN546, AN679, AN693, and others. Follow Application
Notes link from Microchip's home page through to Signal Conditioning/
Measurement, select ADC

> I mean if my analog input was  about 30mV how could it recognize
> this value for example ?

If this is for a temperature sensor, you might be better to amplify
it. Taking the basic range of the ADC as 10-bits in 5V, (we'll say
5.12V to make it clearer), each bit represents 5120mV/1024 or
5mV. If I recall correctly, an LM35 is 10mV per degree, so the
best resolution you could hope for is 1/2 degree. The LM35 is an
"adequate" temperature sensor, and for most general purposes,
0.5 degree would be OK

> here is my block diagram of my circuit
>
> LM35->ampliifer-->ADC---8 bits-->pic--8 bits--->LCD display

I think PIC ADCs are 10-bits minimum

> I am wondering if it would works without amplifier and ADC ?

Any micro can measure analogue voltages, for example the
Scenix, which has no specialised modules at all, just I/O

http://www.piclist.com/techref/scenix/lib/io/adc_vp.htm

> Any suggestions ?

Google for LM35 ADC PIC

http://www.best-microcontroller-projects.com/temperature-recorder.html

Should be able to find quite a few projects like that. Even
look at AVR examples too for the circuits

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