Multiplexing PT100s

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Hi,

 

I wanted to interface 8 PT100s (2-wire) to ATMega64A. How can I compensate for the "ON" resistance introduced by the MUX? By using a MUX I can just use 3 comparators which will make my PCB smaller.

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If you have to stick with a 2-wire connection then it will be difficult. 

 

What mux were you planning on using?

'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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Depends on how you're reading the Pt100s. If you use a constant current source then read the voltage across the Pt100, then the trick is to ensure the mux is not part of the current path - it only switches voltage. 

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"Dare to be naïve." - Buckminster Fuller

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To the OP - PT100 temperature sensors have a quite small (but linear) temperature coefficient. It is not easy to measure with a low resolution ADC. Note that the ADCs in that diagram, above, are 24 bit! When you add multiplexing, it becomes a really big challenge. Better, 8 interface ICs that you read serially (TWI or SPI).

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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 By using a MUX I can just use 3 comparators which will make my PCB smaller.

will it really make it that much smaller...you invite many headaches....if so, take Kartman's advice, mux only the reading of the voltages

When in the dark remember-the future looks brighter than ever.

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Thank you for the reply everyone.

 

I was thinking of using CD4067 or similar, had a look at ADG708 as it has low "ON" resistance of about  3 ohms but is very expensive while CD4067 has an "ON" resistance of about 56 ohms at 5V. 

 

Kartman wrote:

Depends on how you're reading the Pt100s. If you use a constant current source then read the voltage across the Pt100, then the trick is to ensure the mux is not part of the current path - it only switches voltage. 

 

Yes I am switching the voltage, this is what I am trying to do. 

Mulitplexed PT100s

 

Yes if it is getting complicated I might opt out of it but eventually I will have to try this as the number of sensors will keep increasing.

 

Last Edited: Mon. Oct 30, 2017 - 05:37 AM
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What is the 4052 feeding? More importantly, what impedance is it feeding?

'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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So why are you concerned about the on resistance of the mux? The source is low impedance and the adc input is high impedance. 60 ohms vs 600MOhm is not much is it? The humble 4051/2/3 or 4067 is most likely more than adequate for your task.

The schematic says Pt1000. And why a 100uF cap?

Last Edited: Mon. Oct 30, 2017 - 07:12 AM
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PT100 or PT1000 has very few ohms delta for temperature change and the impedance for the digital mux will give you a big error for measure.

use a bridge methode measure with oamp and after mux the output of oamp.

Thierry

Thierry Pottier

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There should be more wires going to the sensor through the mux...your current drive, & the sense lines (+ and -).

 

Your arrangement will give pretty horrible accuracy.  If you are only trying for low accuracy, just use an LM35, or thermistor.

 

 

http://www.ti.com/lit/ug/slau520a/slau520a.pdf

 

http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf

When in the dark remember-the future looks brighter than ever.

Last Edited: Mon. Oct 30, 2017 - 08:36 AM
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avrcandies wrote:
If you are only trying for low accuracy, just use an LM35, or thermistor

or other cheap sensor

 

Digital sensors often have bus communication - so no need for any (hardware) multiplexing!

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Kartman wrote:
So why are you concerned about the on resistance of the mux? The source is low impedance and the adc input is high impedance. 60 ohms vs 600MOhm is not much is it? The humble 4051/2/3 or 4067 is most likely more than adequate for your task. The schematic says Pt1000. And why a 100uF cap?

 

Yeah I will go ahead with it. The diagram i posted is from the article I am using to interface the PT100 sensor. 

https://learn.openenergymonitor.org/electricity-monitoring/temperature/rtd-temperature-sensing?redirected=true

 

 

TPE wrote:

PT100 or PT1000 has very few ohms delta for temperature change and the impedance for the digital mux will give you a big error for measure.

 

 

AVR

Yes, I was afraid of exactly this, hence I started this post.

 

avrcandices wrote:

 If you are only trying for low accuracy, just use an LM35, or thermistor.

 

I am using LM35 currently. The problem is that they are not easily available in the market whereas PT100 is available everywhere, so I decided to switch.

Thank you for the links, they are brilliant, specially for a beginner like me. 

 

I will post on this when I get my results. 
 

Thank you all. 

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What kind of system are you building?

How far are the sensor apart?

 

You might want to consider to make "smart" sensors. Just use an AVR for each sensor and do (long) wires with RS485 (modbus) or similar.

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com