Analog circuit design steps

Break it down into small pieces.

What's needed for an LVDT? You need some kind of oscillator to excite it. What frequency? How much power driving what load? You need something to read the two output coils (If you don't know how an LVDT works, you need to figure that out first). Is one circuit needed to read the coils in series? Two circuits and a summing circuit? I'm sure there are many ways. What is the simplest way to get the results you need? Results mean resolution, stability and repeatability. Next you need a demod circuit to convert the AC output of rhe LVDT to DC.

What do you do with the DC voltage from the LVDT circuits? You mention RS-485 output, so you'll need to digitize the DC signal using an analog-digital converter. How many bits resolution do you need? That's back to the overal resolution you need. If 8 or 10 bits resolution are enough, you can probably do it in the microcontroller. If you need more resolution, you'll need an external ADC. Don't understand how resolution of the LVDT relates to bits of resolution? Guess what...it's home work time. You'll also need to think about how fast you need to make measurements - this may control the ADC chip you pick and interface to the micro.

How will you general the 4-20mA current loop? Analog from the DC voltage of the LVDT stage? An DAC output from the micro?

omartaoufik said:

i am new to analog design and i was assigned to do an analog design to sensor interfaces(LVDT , 4-20mA , differential 485)

Click to expand...

This would start and likely progress much better if you had a data sheet link or two. I see LVDT followed by 4-20 mA and mention of 485, so what is needed would break things as mentioned and get a flow of what does what. Then the big picture of the main goal.

Ron