Current to voltage convertion

Hello Carlos,

0.01ohm sense resistor is pretty small to try to measure 4-20mA.

0.01 ohms * 4ma = 40uV. 40uV * 20 = 800uV.

0.01ohms * 20mA = 200uV. 200uV * 20 = 4mV

So the measured signal will be 800uV to 4mV.

The maximum offset voltage for the system is ±1mV, which means a possible ±20mV on the output due to the offset voltage.

So you signal could be buried in the offset voltage if the offset is negative.

Try 10 ohms of Rsense for a 5V supply.

Also, the input pins cannot be more than -22V to +60V from the GND pin.

Please provide a complete schematic of your circuit. I am unclear of your statement "but the IC is not responding when i plug the 4-20mA terminals of a Fluke into the IC pins". How are you measuring?

Regards,

Hi Paul,
Thanks to reply so soon, I'm using a Fluke calibrator to provide the 4-20mA, certainly the 0.01ohms resistor was to small to measure the 4-20mA current.
I changed the resistor to 10 ohms and the result was better but i'm having this other problem:
At 4mA in the input, the output is 3.25v and when the input is >= 13mA the output is always at 4.91v, i think this must be because the internal Preamplifier and the Output Buffer are amplifying the signal and this it can't be greater than the 5v of the power supply, am i right?
How can i fix this?

When i said that the IC was not responding maybe was because the output was so small to see it clearly with a Tester or the oscilloscope, my mistake.

Here is attached the schematic of the circuit (I don't have the Altium's library for the component so i replaced it with another component).

I expect your promt response.
RegardsLMP8601.pdf

Hi Carlos,

4mA * 10ohm = 400mV, 400mV * 20 = 8V

Simply, the theoretical 8V output cannot exceed the 5V supply voltage. Where would the "extra" 3V come from??? You need to reduce the sensitivity by reducing the size of the sense resistor.

So lets make 4.5V the max output at 20mA. 4.5V / 20 = 225mV,  225mV / 20mA = 11.25Ohms. 11.3 ohms is the nearest 1% standard value. Or use two 22 ohms in parallel.

Another issue is that your current source is "floating". The LMP8601 (and most other inst amps) are not capable of a truly "floating" input.

The common mode voltage (the voltage common to both input pins) must be within -22 to +60V from the GND pin. There must be a DC path from one of the input pins to GND. Try placing a 100K resistor from +IN to GND to provide a DC path.

Regards,

Hi Carlos,

My previous post was in error..I was off by a decade...The caffeine did not kick in yet..

Lets try again..

4mA * 10ohm = 40mV, 40mV * 20 = 0.8V

With the OFFSET pin tied to V+, the zero output level will be 2.5V

0.8V + 2.5V = 3.3V

The output will "rail" at (2.5V/20=125mV, 125mV/10ohms) = 12.5mA

Taking into account the offset error, resistor tolerance, and any error in the 5V supply voltage, your measurements are correct.

If you want to get the full output swing and you do not need to measure negative currents, you can ground the OFFSET pin so that the output is zero-referenced (0mA = 0V). The output "zero" reference will be one-half the voltage applied to the OFFSET pin.

If you want to get the full dynamic range, use a 11 ohm resistor and ground the OFFSET pin.

Another issue is that your current source is "floating" as shown in the schematic. The LMP8601 (and most other inst amps) are not capable of a truly "floating" input.

The common mode voltage (the voltage common to both input pins) must be within -22 to +60V from the GND pin. There must be a DC path from one of the input pins to GND. Try placing a 10K resistor from +IN to GND to provide a DC path.

Regards,

Paul Grohe

Integrated Signal Chain Applications, SVA