Help understanding resistors in front of inst. amp - temperature sensor

[mspagon]

I'm a complete beginner in the design area... but I have this circuit that uses an RTD (a resistor that is relatively linear and proportional with temperature).

I've done a week of reading on bridge networks, op-amp configurations, instrumentation amps, and various other topics, but I don't see a bridge network here!

Can someone help me to understand what the purpose of the configuration of the resistors and how to determine voltage levels between them? I have no idea!

Much thanks and appreciation,

Michael

 

[FvM]

I fear, it can't be fully understood because it looks like an erroneously designed 3-wire RTD amplifier.

For an ideal 3-wire amplifier, the output signal doesn't change for equal wire resistances at both RTD terminals. It measures the voltage drop across one wire and compensates boths, assuming they are equal.

 

[mspagon]

actually, I think the configuration is correct, but I don't know what the 10k and 100k resistors are for... the bridge is before the 10k and 100k, those resistors are after the "input"... so why have a voltage divider on the input of the inst. amp?

If you match the picture b) to my circuit everything aligns.

http://www.rdfcorp.com/anotes/pa-rtd/pa-rtd_04.shtml

 

[FvM]

Calculating the picture b) circuit shows that wire resistance only cancels for Rt = R3. So if you use it with a Pt1000 sensor and R3 = 1k, you get a certain reduction of wire resistance influence, but no cancellation. For Pt100, the effect is only minimal. Thus I don't recognize it as a correctly designed three-wire circuit.

The 10k resistors can be understood as protection (overvoltage, ESD) means, the purpose of the 100 k resistors isn't obvious and probably not well considered.

As a general comment, as with all voltage divider RTD circuits (in contrast to current source excitation), you get a non-linear transfer function that has to be corrected in software.