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The MAX31865 is an RTD to digital converter IC. I want to measure both PT100 and PT1000 on the fly without changing REF resistor.
A 430ohm resistor is need for PT100 and a 4.3kohm resistor is need for PT1000.

I want to design a circuit that is capable of measuring both PT100 and PT1000 without soldering another REF resistor. How can I make an extra circuit or something for this purpose?

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JRE
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Berker Işık
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  • Looks like current trough that resistor is always flowing in the same direction, from BIAS to Isensor. You could try switching between multiple resistors using a small MOSFET like 2N7002. If the couple of ohm on resistance does not affect your required precision too much. – Unimportant Jul 25 '21 at 12:26
  • Can you post an example circuit? – Berker Işık Jul 25 '21 at 12:39
  • Example borads use %0.1 tolerance resistors. I'm not sure RDS of Mosfets can stay constant? – Berker Işık Jul 25 '21 at 12:41

2 Answers2

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You could use an analog SPDT switch (or a SPST switch with the 4.3K paralleled).

For example, the NLAS5223

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You should do a detailed calculation of the effects of series resistance, series resistance variation (eg. with temperature) and leakage and compare it with your error budget.

Alternately, a small telecom-style relay, preferably a bistable type to minimize thermal EMFs, could be used. Resistance and leakage will not likely be of concern but the driving is a bit more complex and contact reliability may be of issue under some conditions.

Spehro Pefhany
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enter image description here

Figure 1. RREF is not referenced to ground or VDD so this isn't going to be easy.

The datasheet opening paragraph says:

The MAX31865 is an easy-to-use resistance-to-digital converter optimized for platinum resistance temperature detectors (RTDs). An external resistor sets the sensitivity for the RTD being used and a precision delta-sigma ADC converts the ratio of the RTD resistance to the reference resistance into digital form.

... so you have to use the 430 Ω or 4300 Ω reference as appropriate.

If a jumper was an option the sensible approach might be to put both in with the jumper selecting which to use. An alternative would be to solder in the 4300 Ω resistor with a jumper connecting a 477.77 Ω resistor in parallel (the combination giving 430 Ω) - but where are you going to get one of those?

I don't think you're going to find an easy solution to this due to the precision required for the reference and the fact that neither end of RREF is connected to ground.

A miniature relay is probably your best option.

Transistor
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    If it needs to be switchable electronically, an electromechanical DPDT relay would probably work too. Make sure it's rated for low-current use; you probably want one with gold or rhodium contacts, not tungsten. – Hearth Jul 25 '21 at 13:30