I have a project that needs to convert a 0-60K Ohm resistance to an analog PLC input. I have found several signal conditioners for this, but the max resistance was always the standard 10K or 20K maximum resistance. Does anyone know of one that goes higher? I was thinking of putting another resistor in parallel, but then the curve will no longer be linear. I can't put any real voltage on the line (>24Vdc) because of the hazard to personnel.

Anyone have any other ideas?

Thanks

 

Get a dual OPAMP and set one side up as a 100uA constant current source and the other side as a voltage follower. The CC source will give you 0-6V for the 0-60k resistor and the voltage follower will buffer the voltage for the low Z analog input.

Regards
cww

 

Most of the resistance to Current transducers are actually Voltage to Current taking the slidewire voltage and converting to current. There's no reason why they shouldn't work with a much higher resistance. or do you just have two wires available?

Roy

 

An Analog Devices 7B isolator (or one of the clones from Dataforth or someone else) will give you up to 2M ohms of input resistance (if you choose the right model).

To get a order of magnitude or more better than that, I think you will need a custom circuit based on one or more instrument op-amps. Noise sensitivity and other issues will likely be a problem, so conventional industrial wiring and installation practices might cause a problem. Location, shielding, wiring, and other factors will be very important.

 

Can you let us know more about the application/sensor? Is this a thermistor, high conductivity liquid, potentiometer with 3-wires, some-type of impedance checking circuit, etc?

Acromag has off the shelf signal conditioners that will give you options. And, if your signal will be non-linear, not a problem. The products have configurable linearizers or math functions if needed.

Kind Regards,

Donald Lupo
dlupo [at] acromag.com