pt100 and 4051

[Marco Trapanese]

Hello,

I want to acquire 8 pt100 in three-wires configuration. I'm going to use
a single LTC2402 as described here:

http://cds.linear.com/docs/Design Note/dn237f.pdf

so I need to multiplex the 24 incoming wires.
For example I could use three 4051.

The Rds_on resistance is quite high but it shouldn't be a big deal
because the current flowing is very small.

I'm afraid about the very high delta among channels... the datasheet
says it can reach 15 ohm when Vdd = 5V. Too much!

Do you know any other analog multiplexer with a matched Rds_on?
Or do you recommend a completely different design?

I cannot use relays for this purpose.

Thanks in advance
Marco

 

[whit3rd]

I want to acquire 8 pt100 in three-wires configuration. ...
so I need to multiplex the 24 incoming wires.

So, your intent is to switch the excitation current in one lead of the
RTDs but to leave the other lead's excitation current common
between the eight units? That takes an 8-to-1 switch, then you
use the other two 8-to-1 switches
to probe the third wire and the common node of the
unswitched excitation wiring.

There's no need here (unless your voltage sense amp bias current
draw is significant) for the switches to match in resistance.
Only the excitation wires on the two poles of the RTDs have
to be matched (as in all three-wire resistance schemes).

If, rather than leaving one excitation (current-carrying) wire common,
you wished also to switch it, the problem of switch resistance mismatch
does show up, and the multiplexer has to grow to four-wire
complexity (i.e. you'd use four 8-to-1 switches), even though the device
wiring stays in the three-wire configuration.

 

[Marco Trapanese]

Il 13/01/2013 18:22, John Larkin ha scritto:

HC4051s are fine. Arrange the circuit so that the switch resistance doesn't
matter. You'll need three mux chips.

Here's a 4-wire mux'd RTD thing, similar concept:

https://dl.dropbox.com/u/53724080/Circuits/Thermal/V470_RTDs.pdf

R324 is a Susumu thinfilm resistor, and everything is ratiometric off that.

I'm trying to fully understand your schematic.
One channel of AD7793 reads the mux'd RTD's signals while the second
channel measures the voltage drop across R324, that is the current
flowing into the selected RTD.

In this way, the Rds_on of any channel of the three 4051 is different
doesn't matter at all?

Thanks for your hint
Marco

 

[Marco Trapanese]

Il 13/01/2013 19:06, whit3rd ha scritto:

So, your intent is to switch the excitation current in one lead of the
RTDs but to leave the other lead's excitation current common
between the eight units?

No, I was mean to use a switch for each lead. So three 8-to-1 switches.

That takes an 8-to-1 switch, then you
use the other two 8-to-1 switches
to probe the third wire and the common node of the
unswitched excitation wiring.

I will try to draw a schematic and I will post here to be sure I
understood correctly.

There's no need here (unless your voltage sense amp bias current
draw is significant) for the switches to match in resistance.
Only the excitation wires on the two poles of the RTDs have
to be matched (as in all three-wire resistance schemes).

In your solution doesn't matter, but in mine does.

If, rather than leaving one excitation (current-carrying) wire common,
you wished also to switch it, the problem of switch resistance mismatch
does show up, and the multiplexer has to grow to four-wire
complexity (i.e. you'd use four 8-to-1 switches), even though the device
wiring stays in the three-wire configuration.

Yes, now I understand my error.

Thanks
Marco

 

[Marco Trapanese]

Il 14/01/2013 00:01, John Larkin ha scritto:

Hardly at all, not enough to matter. Only one switch needs to carry current, and
it just needs to be short-term stable.

Right, the other two legs doesn't carry any current (ok, it's very very
small -> negligible)

What sort of RTD current did you have in mind? 200 uA is a little low for my
taste... only makes 20 mV across 100 ohms.

I usually power my RTDs with 500 uA. No evidence of self-heating and the
voltage is not very difficult to read using a 24-bit converter and a
good layout.

In this particular case I may rise the current over 1 mA because I can
power each RTD for a short time. I need to get a reading every several
minutes.

Thanks
Marco