Electronics > Beginners
Multiplexing RTDs with MAX31865
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honeybadger:
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?
honeybadger:
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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