Struggling to select the correct Analog Switch

I wouldn't want to break any circuit the existing Pt100 is in, purely because its resistance is so low compared to the actual resistance that would be introduced by your switching elements. That would have a significant impact on the measured temperature by both systems. With a nominal resistance of 100-ohms at 25C, it really won't be happy if you added another 20-ohm (typ at 10mA) for that particular switch for example in the illustrated two-wire connection method.

For slow-speed switching, perhaps you can get away with a nice gold-plated signal relay depending on the current. That would have much lower contact resistance and thus influence on temperature.

I would still suggest reverse engineering the current circuit more. Many Pt100-based devices use three-wire or four-wire connections to negate lead resistance for higher accuracy. In the case of four-wire, you could conceivably use higher resistance switches as long as it's being compensated for by the four-wire connection (but watch the limits of compensation by the circuit). If it's three-wire, then likely it will be running a common ground side, so switching one-side and having compensation on that side is a possibility too.

Instead, if you are able to reverse-engineer it further, then that would be even better. At a guess, most Pt100 RTDs are likely to be driven like a DMM would, with a constant current through its drive pair. A voltage is read back through a sense pair (depending on how it is configured, in two-wire, they are the same; in three-wire, they are separated on one side and commoned on the other; in four-wire, they are all independent). If you can know what this constant current is, then the Pt100 will have a voltage on its sense connections that will tell you the temperature - from there, I would probably choose a low input offset current op-amp and use it as a voltage buffer into your own system. That would present a very high impedance (thus affecting the current circuit very little) and avoid any loading effects. You will have to know what the measurement circuit's voltage is to avoid burning out an opamp, but it's likely to be small because they're not looking for a high current through the RTD, as its self heating would affect the result.

- Gough

Gough Lui said:

I would probably choose a low input offset current op-amp and use it as a voltage buffer into your own system. That would present a very high impedance (thus affecting the current circuit very little) and avoid any loading effects.

I cannot quite see how this works. If possible, could you sketch it - I like pictures

Gough Lui said:

I would probably choose a low input offset current op-amp and use it as a voltage buffer into your own system. That would present a very high impedance (thus affecting the current circuit very little) and avoid any loading effects.

I cannot quite see how this works. If possible, could you sketch it - I like pictures