I'm currently designing (or at least trying to) a RTD amplifier with ICL and current outputs.

What I've got so far is a working RTD and ICL schemes, but I still struggle to design a proper converter. The main issue is that the load impedance can reach up to 600 Ω. Input voltage range doesn't really matter, I can deal with that by adjusting my RTD, but as a complete amateur I have no idea how to design such a converter. I've already found one solution that I like on Stack Exchange, but even though in theory it should tolerate up to 1100 Ω, it clearly doesn't.

Here's a link to the topic I'm talking about:

Converter that I've found

Also, for the curious people (to actually confirm that I'm a complete amateur in electronics), here's what I've got so far:


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    \$\begingroup\$ You may want to have a current mirror at the output so you can get rid of the 250Ω resistor in the load circuit – 600Ω output impedance is already a lot. \$\endgroup\$ – Janka Nov 25 '18 at 14:19
  • \$\begingroup\$ Oh, forgot to mention that the converter on the second scheme is just a test one, that's why I needed help with the converter from the first scheme. \$\endgroup\$ – Kholgar Nov 25 '18 at 17:31
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    \$\begingroup\$ Can you try to ask a specific question? Right now your post seems like a very general, very broad, request for comments. \$\endgroup\$ – Elliot Alderson Nov 26 '18 at 0:24
  • \$\begingroup\$ Stupid me forgot about droput voltages on OPamp and that's why the converter from the first scheme didn't work as intended. Fortunatelly, Andy aka has already solved my problem. Warned you that I'm a complete amateur in electronics, right? \$\endgroup\$ – Kholgar Nov 26 '18 at 9:00

in theory it should tolerate up to 1100 ohms, it clearly doesn't

20 mA through 1100 ohms requires a voltage across it of 22 volts - your power supply is only 10 volts so this will never happen. You need to use power rails that are a few volts greater than what is needed at the maximum current and maximum load resistance to accommodate op-amps not being able to drive their outputs to the power rails. Then you need a few volts more to accommodate the voltage dropped across the sense resistor (R3 in the top circuit in your question). Trust in Ohm's law!

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