Voltage concern on 4-20mA OpAmp input?

[zbrown78]

A project that I am working on requires adding inputs to read of a 4-20 milliamp circuit produced from a loop-powered sensor. One way that many controller manufactures do this is by placing a 250 Ohm shunt resistor across the two inputs to the controller to get a 1-5volt signal. (which is normally a 0-5volt input) The 1-5volts is then scaled accordingly to give a valid range of the sensor.
Question: If the OpAmp that I am using only allows inputs to rise to Vcc+.3( in my case, 5 volts Vcc...so..... 5.3VDCmax), how do you read a sensor with 24 VDC loop power? The is extremely common in building controls work. Is the voltage drop across the shunt resistor the only thing that the OpAmp sees or will the 24volt loop-power burn it up? Several controller cut sheets that I have refered to make no mention of loop-power voltage. I was planning on using a MAXIM Rail-to-Rail OpAmp connected to a precision ADC going to my micro. I DON'T WANT TO COOK MY MICRO. Thanks, Z

 

[kender]

Is there anything preventing you from powering the OpAmp with +24V? Then you can scale the output to match the input range of the A/D.

- Nick

P.S. Here's another good place for sensor-related questions: **broken link removed**

 

[FvM]

The voltage drop isn't actually related to the loop power, because the loop powered sensor acts as a constant current source.
So everything is fine in normal operation.

Faulty operation is different topic. You should generally be aware of possible sensor shorts, sourcing full 24V with a most likely
high short circuit current to your current input. Obviously overload protection is advisable, at least for a professional product. It's
rather easy to protect the buffer amplifier with a series resistance. The shunt resistor itself must have a 2.5W rating to withstand 24V.
Several protection concepts have been implemented in automation, e.g. PTC fuses or electronic current limit. But it's not easy.