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

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#### mspagon

##### Junior Member level 3

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

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

### mspagon

Points: 2
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

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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.

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