Re: Thermocouple mux 16 ch(4067) + A/D max187

[Marc F Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Sat, 10 Dec 2005 09:33:52 GMT, "max" <maxitubo@xxxxxxxxxxx> wrote in
message  <4ixmf.29739$S6.548120@xxxxxxxxxxxxxxxxxx>:

>Hi Marc,
>
>Thanks forthe answer.
>
>Yes the AD 595 contain an Alarm circuit that control if thare is a open
>TC(2>wire open) but isn't necessary to use it.
>
>I tough to switch only the positive laed of TC and connect all 16 negative
>lead togheter with the common ground of the circuit ..?ve seen that some
>rotary selector for TC switch only 1 wire and the other are connect
>togheter to gound. I?ve seen bad?
>This can create some error?

This is OK if you are not measuring temperatures below 0 C (The "ice point'
is typically set to 0 volts) *AND* you have adequate input protection.

>My problem is that i don't know how to control via Parallel port 2 4067
>mux.

The solution is simple: both multiplexors are controlled by the _same_ 4-bit
signal so both thermocouple wires are switched at the same time.

Are you planning to use a single (+ only) or bipolar ( + and -) power
supply? The digital switching is slow enough that it doesn't add significant
noise and the AD595 is rated for a single 5.0 vdc supply so one could use a
single 5vdc supply for both analog and digital.  Conventionally, one uses
bipolar power supplies if temperatures below freezing need to be measured
because freezing (0 C) is often set to 0 volts.

It's a good idea to have overvoltage protection at least at the outputs of
the multiplexors. If you were following the multiplexors with a differential
amplifier as is conventional, I would suggest that you put 100Kohm resistors
in series with each thermocouple lead between the lead and the multiplexor
input. But I don't how those series resistors would affect the thermocouple
error circuit in the AD595.

For bi-polar supplies, use two zeners in 'reverse parallel' (one in each
polarity from signal to ground) at each output (4 zeners total). Together
with the 100Kohm input resistors, they provide good protection.

On pc boards used for sensor input and external control I like to put the
first IC's in the signal path leading onto and from the board in sockets.
That way the IC's can be easily replaced if they are damaged for whatever
reason. These IC's are cheap, so it is a practical solution.

An input multiplexor board along this design is needed for the supplemental,
16-bit-resolution analog input on the hard-wired lighting controller I
describe at http://www.econtrol.org/16-pos_ak.htm, so, time permitting,  I
will put together a circuit diagram and pc board layout and post  to the web
site later this weekend.

Hope This ... Marc
Marc_F_Hult
www.ECOntrol.org