4-20mA Application: need some advice, please

[andrea@xxxxxxxxxxxxxxxx]

Hi all,
I need to monitor six 4-20mA sensors at once, with a resolution of 16 bits.
It is half a hobby project and half a favour to my grandfather, although
I'd also like to put it in my curriculum vitae, for further advantage. I
am not an engineer, I am a self-taught programmer, but the more stuff I add
to my resume, the better, ain't it? ;) So, I think it's now time to start:
I'd like to use one single 12V or 24V battery to power all 6 sensors (one
at a time), convert the 4-20mA signal to a voltage, channel it (through a
multiplexer) into an ADC and read and log the data via a microcontroller.

Could you give me some advice if I'm doing right or wrong? And why?

a) For the battery part, I thought about using one or two 12V sealed lead
battery or two PP3 9V lithium 1200mA/H batteries (a friend told me they
exist and look like normal 9V batteries, although they have x 10 mAH!).

b) For the input stages, I'm thinking about using a scheme like this:

   Battery +24V or +18V (not yet decided)
     |
    RES (to limit max current in case of Sensor's short circuit)
     |
===Sensor (4-20mA type)
     |
   MOSFET-- on/off control (would a relay be better? why?)
     |
     *------*--- to Analog MUX / ADC (Sensor's 0..20mA becomes e.g. 0..5V)
     |      |
 5V Zener  RES (to convert current to voltage)
     |      |
     *------*--- ground

I saw the RCV420 by Texas Instruments/Burr Brown, but it costs $22 here
in Italy! And anyway I'm not sure it would really help in my application.

The MOSFET is there to "isolate" every other sensor, so that only one is
getting current at a time. However, I'm not really expert of high-side,
P-Channel MOSFETs and at first I thought about a N-Channel solution, but
on the bottom, between ground and Zener/Resistor. The N-Channel solution
would cause a measuring error though, unless I can take it into account
on the ADC. I think the P-Channel solution may be more "proper" anyway.

The purpose of the bottom Resistor is easy to understand, it will convert
the current into a voltage. The Zener is there to make sure that voltage
doesn't go too much beyond 5V even in case of a Sensor short circuit (the
current will be limited anyway by the top Resistor).

Would a Transil diode instead of the Zener be really useful as protection?
I am thinking about some hundreds of meters of cable which, due to its own
induction, could kick very hard when the MOSFET turns on. Of course, if a
cheaper Zener solution is as good in practice, why waste a Transil there?
A reason may be electrostatic interference or nearby lighting strikes,
would a Transil be a more valid choice than a Zener then (I know that if
the lighting strikes right on the cable, nothing can be saved anyway!).

Now I (think) I could feed directly an ADC input (should I buffer it with
an op-amp? Why? The impedance is already pretty low, and while the ADC
input may draw bias current, I think I can calibrate the whole system at
the digital output, cannot I? Or the ADC input bias current will vary, thus
introducing an error in the measure? I'd like to get near 16bit resolution).

I saw some interesting ADC's from Texas Instruments which contain also an
analog multiplexer. For example the ADS1243 IC, which contains a 8-MUX, a
buffer and even a programmable gain amplifier, and it's even 24 full bits!

Thank you very much for any useful advice,
--
Andrea