How to Start Analog Circuit Design for LVDT, 4-20mA, RS-485 Sensor Interfaces?

Hello everybody i am new to analog design and i was assigned to do an analog design to sensor interfaces(LVDT , 4-20mA , differential 485) . the problem i am facin g right know is that i don't know exactly from where to start, (for example i don't know if i need a filter or not, and if i need one then what  type of filter should i design) it's like i don't know the right steps or procedure to design an analog cicruit,can you help me with ideas even a book or an article can help 

Analog Devices make a couple of LVDT interface chips (AD598, 698), they have good design information in the data sheets. Filtering  is discussed in the notes. I have used one of the chips in an LVDT system quite successfully. You could design your own, but then you will need to characterize it, which ADI have already done for you.The band width may be limited by the application, you don't say what the mechanical system on the LVDT is.
You need to establish that first, since it determines things like the required bandwidth, dynamic range, signal to noise,etc., and antialiasing filter requirements.Start with a conceptual  block diagram of the whole system, then fill it out with more specific details. The transducer front end will be followed by both analogue (4-20mA) and digital (RS485). The current loop will require appropriate level shifting from the front end device.  The RS485 implies digital output, so you will have an ADC  as well, so a micro is likely to be needed.Try a book like "the art of electronics", the 3rd edition is most up to date. There is also an extraordinary amount of information on the web. Chip manufacturers often have detailed design notes.Without more details hard to know what else to say, except that there are many ways of approaching a problem, in the end it is usually experience which gets you on the right track in the shortest time.  Not having that, if you can modularize the system before you commit yourself to a final design, it is possible to optimize each part of it, swapping out or working on those which don't meet the specs.  So breadboard each section until you get it right. Do your maths, essential to meet the design specs, and field test it as well,cheers,Richard

I am not asking about a certain circuit, my question is general, let's take an example, while designing a circuit i don't know if i need a filter or not, and if i figured out that i need a filter then what are the filter specs ? it's because i am new to this type of design, 

OmarWelcome to my world! You are asking many questions here- of course when you start out you don't know what you don't know.The first issue is what kind of sensor-  RTD, Thermocouple, strain gauge etc.. That will determine if you need a filter or not. The next issue is what sort of output do you need, and whether or not the output communication is bidirectional. As Richard mentioned 4-20mA is an analog technique and depending on what your needs are, your design can remain completely analog. You would need to add HART (a protocol superimposed on the 4-20mA) to get bidirectional communications in this scenario. LVDT can stand for "Linear Variable Differential Transformer" or "Low Voltage Differential Transmission". Both would be valid in this context, the former being an input transducer and the latter a communications medium. RS485 is differential, but more importantly is half-duplex. You would need RS422 for differential duplex communications.The last issue is what sort of processing you need to do to the signal and that would determine whether you need digital processing. There are no absolutes- it depends on your experience, preferences, support and cost requirements.I attempted to document all I know about 4-20mA in an article I wrote for  Circuit Cellar, Issue 241, August 2010 "The 4-20mA Current Loop". (CC's business model is to charge for archived articles.)But you should know there are often ICs that for subsystems. There are A/D converters that make provision for thermocouples and RTDs and a host of others- try digging through offerings from  Analog Devices and Linear Tech (now Analog Devices as well) as well as Maxim. There are also ICs that will translate sensors to 4-20mA. TI and Maxim do these very well- take a look at TI's XTR111 and there are several others.Finally companies also publish app notes and reference designs. Some are pretty comprehensive like this one from TI. Take a look at this reference design for RTD to 4-20mA

To answer your question do you need a filter or not:  That is almost always a YES  because of the world of wireless, these RF signals will tend to interfere and cause all kinds of "nasties".  The general rule of thumb is your bandwidth should be set to what you need and nothing more.