3 wire RTD with current source and INA

Question

How would I connect a 3 wire RTD using current source and a instrumental amplifier for the output?

Answer 1

If you have only one current source it's a little difficult with an inamp because you need a different gain from each input.

For example, in this circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Va = I(RTD + Rlead)

Vb = I(RTD + 2Rlead)

So you could have Vout = 2Va - Vb = I*RTD

What's typically done is to add another matched current source.

^{simulate this circuit}

So Va = I(RTD + 2*Rlead)

Vb = I(2*Rlead)

And Vout = (Va - Vb) = I(RTD)

So you would just connect the instrumentation amplifier inputs to Va (+) and Vb (-)

Of course you could also add a gain of +2 op-amp to the first circuit before the instrumentation amplifier (+) input- an ordinary precision op-amp and two equal-value resistors- which retains the high input impedance of the inamp but adds another active circuit and another source of drift. The two-current source solution is relatively insensitive to matching errors in the left current source if Rlead << RTD resistances.

Answer 2

Do this: Source:

https://electronics.stackexchange.com/q/304817

But instead of using a diff amp, connect the instrumentation amp positive and negative to Vin1 and Vin2.