Author Topic: Multiplexing RTDs with MAX31865  (Read 1413 times)

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Offline honeybadgerTopic starter

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  • Country: cz
Multiplexing RTDs with MAX31865
« on: February 14, 2020, 02:54:08 pm »
Hi,
for my next project I need to measure up to 8 temperatures with a reasonable accuracy. I don't want to mess with thermocouples because wires will be few meters long. DS18B20 has a 125°C maximum, I need 150-180°C.
I guess PT100/PT1000 is the best/only choice and MAX31865 is relatively cheap and available. 3-wire method to get the accuracy and low wire count.

Because I don't want to use 8 separate MAX31865 chips I need to multiplex the RTDs somehow - don't care about slow measurement, 30s sampling rate is more than enough.

  • What chip would you recommend to do the multiplexing? CD4097B for example?

  • I am not sure about the circuit design. Should I use a one MUX to switch the FORCE+, other chip to switch the RTDIN+, etc.?
    I think this is not a good idea because Rdson is matched inside the chip but every chip might be different and with 3-wire you rely on the same resistance of all 3 wires.
    So would be a better idea to use for example CD4066B as a switch and use a separate chip for each RTD so all 3 wires go through the same chip?

  • I am not sure about that but is it necessary to MUX the FORCE-/RTDIN- line? They are connected together in 3-wire mode. Would the MUXing be better for, I don't know, interference protection?




 

Offline honeybadgerTopic starter

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  • Posts: 42
  • Country: cz
Re: Multiplexing RTDs with MAX31865
« Reply #1 on: February 15, 2020, 01:14:06 am »
Update:
I do need all 3 wires to be MUXed and all 3 wires needs to go through the same chip - that is the price I have to pay for 3-wire PT100.

NX3L4053PW - 3.3V , Ron 1ohm - one for each two PT100.

TS3A44159 - 3.3V, Ron 0.45ohm - NO, it does not have EN pin!

TMUX1574- 5V, Ron 2ohm - one for each two PT100.
TMUX1511 - 5V, Ron 2ohm - one for each PT100.

Any other suggestions for  3/4 pole switch with low Ron, preferably double throw (so I can hook two PT100) and with EN pin (so I can disconnect both)?
 


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