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Pt 200e Reading

Are the connections for reading a temperature sensor using CCS( constant current source) really like in my picture? The analog and the current source seem to be shorted.

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  • \$\begingroup\$ What are you asking ? \$\endgroup\$ – efox29 May 23 '15 at 5:18
  • \$\begingroup\$ Ccs and Analog input pins seem shorted i want to know is it a problem for 1 ma current flow or not? \$\endgroup\$ – Soso May 23 '15 at 5:22
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If I understand your question correctly, you are asking why the analog and the current source input are connected this way.

The answer is: your drawing is correct, but makes it hard to understand why this setup is used (in the drawing the lines are not connected right at the sensor).

The idea behind using four wires is to eliminate errors from voltage drop in longer leads to the sensor. You can imagine the wires to the sensor as resistors (RLx in the schematic). The sensor itself is RT. The measurement current Iex will always create a small error voltage in RL1 and RL4 (you can't have wires without any resistance), but by using separate voltage sensing lines V0 will be the pure sensor signal (voltage over RT).

diagram

The resistances RL2 and RL3 don't really matter because the voltage input has a really high input impedance, so the current in the voltage sensing lines is close to zero.

Of course it is important to connect the voltage sensing lines to the current lines right at the sensor for this setup to work.

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