I have 4 LVDT sensors. Each sensor has power supply +15V, -15V, Ground and sensor analogue output -10V -> +10V. Each sensor takes ca. 35mA.
I have one voltage converter that converts 0-5V to -15V, Ground, +15V
I have one ADC that can take -10V -> +10V.
All that will be connected to arduino -> PC USB port
I want to switch between the sensors. All sensors are powered off. I power on one sensor read the LVDT value and power it off. I continue with reading the second sensor and so on.
How can I achieve that?
Shall I use relay, multiplexer, analogue switch, transistor or anything else?
Do I need to disconnect all the power and signal or is it fine just to disconnect only some of it and keep the rest connected all the time?
I cannot keep all the sensor running because they would take too much power.
I have not looked closely at the data sheet but I wonder whether you could use analogue switches such as these to switch the power to the LVDTs and to switch the analogue output:
That's the first one I found that looks as if it may be suitable. I have not searched for alternatives.
svecoldr:
Do I need to disconnect all the power and signal or is it fine just to disconnect only some of it and keep the rest connected all the time?
I'm confused! I understand you want to power only one sensor at a time. What are you thinking of leaving on all the time?
Thanks, I will take a look at it. What i mean is if I need to disconnect power +15v -15v ground and signal or if it is enough if I just disconnect only e.g +15v supply wire to completely "disconnect" the sensor.
Can I for example keep all the sensors connected to ADC and just disconnect power?
Edit: Oh I forgot to mention that dip, sip etc is the only thing I can use. Anything smaller is too small for my hands 
svecoldr:
What i mean is if I need to disconnect power +15v -15v ground and signal or is it is enough if I just disconnect only e.g +15v supply wire to completely "disconnect" the sensor.
You will probably need to disconnect both +15V and -15V to stop a sensor drawing current.
svecoldr:
Can I for example keep all the sensors connected to ADC and just disconnect power?
As you have only one ADC, I understand you will have only one sensor's signal switched to the ADC at any time. You can leave a sensor connected to the ADC when the sensor's power is disconnected. It would be inadvisable to disconnect the power to the ADC while a signal from a sensor is connected to it.
svecoldr:
Edit: Oh I forgot to mention that dip, sip etc is the only thing I can use. Anything smaller is too small for my hands
I had not spotted that package! Perhaps you can find something similar from another manufacturer that can handle the voltage and current requirements. If not, I would resort to using relays to switch the signals and discrete transistor circuits to disconnect power to each sensor.
Depending on your sensor modules, you may power the sensors by 30V, and add an voltage divider for the ground (15V) input, if required. With such an asymmetric supply you have to switch only one power line per module. With a low measuring frequency you can turn on all modules at once (one transistor), measure the values, and turn off all modules again. In either case allow for some initialization delay (settling time) after applying power to a sensor module, before you start reading values.
Use another voltage divider to convert the signal level (0 to 30V) into the range of your Arduino ADC (typically 0 to 5V, or 0 to 3.3V, depending on Vcc and AREF). The Arduino Gnd then connects to the negative supply of the modules (labeld -15V), the module Gnd is not connected (not used).
DrDiettrich:
Depending on your sensor modules, you may power the sensors by 30V, and add an voltage divider for the ground (15V) input, if required. With such an asymmetric supply you have to switch only one power line per module. With a low measuring frequency you can turn on all modules at once (one transistor), measure the values, and turn off all modules again. In either case allow for some initialization delay (settling time) after applying power to a sensor module, before you start reading values.
I guess the sensor's ground connection is connected internally to the metalwork of the sensor.