XTR115: Output is stable at 11mA

Hi,

U15 must not be powered from outside but from pin 8 of U8. Otherwise the return current resulting from the current through R8 has the wrong direction and flows out of pin 3 back to the GND of 3.3V supply. But it must flow out of pin 4 which is only possible if U15 is powered from U8. Have a look at figure 1 of datasheet of XTR115 and see how the individual currents are flowing.

Read also the section "minimum-scale current" of datasheet!

Kai

i did what you suggested but now 10k resistor looks 2.2k at multimeter and output is too much ??

Hi,

what do you mean by "10k resistor looks 2k2 at multimeter"? You cannot measure the resistance of R8 when the circuit is powered. This gives wrong results.

Divide and conquer: Remove U15 completely (!) from U8. Change R8 from 10k to 20k. Connect the left side of R8 to Vref of U8. Connect a 500R burden resistor from "-Loop" to the GND of "+Loop" supply voltage. Take a voltage meter and measure the voltage drop across burden resistor. What do you measure? It should be 2.5V / 20k x 100 x 500R = 6.25V. If you measure this voltage, U8 is still working properly.

Now connect U15 to U8 again. But take care that no current that is originating from the XTR115 flows elsewhere to different circuitry. All these currents must flow back to the XTR115!!! Provide optical isolation for the SDA and SCL lines, if necessary, as shown in figure 3 of datasheet of XTR115.

Kai

No the circuit is not powered of course ! and before doing all you said. if i can not use 10k then i need to use xtr116 for getting 4-20mA right ? because xtr115 gives 2.5V Vref so 2.5V / 20k * 100 = 12.5mA maximum current! and i connect a multimeter to loop- for measuring current is that OK right ?

Hi,

the modification with the remove of U15 was only a test to make clear that your XTR115 is still working and hasn't become damaged during the experimenting.

Take R8=10k in your circuit, if you need. But isolate the circuit of U8 from the rest of the world by using optical isolation, as shown in figure 3 of datasheet. The vagabonding return currents are your problem!

Kai

Something happend and i cant measure any current at -loop but if i read voltage at the same time between -loop and +loop  i can read 12.5mA at loop-

Is this Ok ?

I saw something at forum and it makes me start thinking. Only inputs i have are loop+ and loop-. I thought Iret provides the ground. Is that true ? or do i need an other ground entrence ? or isolating loop- to a ground ?