I recently purchased a MikroElektronika 4-20mA T Click MCU Add On Board MIKROE-1296, which features a MCP4921 DAC and XTR116 current loop transmitter, to control a current between 4-20mA.
However, I can't seem to get it to control the current to less than about 4.4mA despite being rated by MikroElektronika to have the current down to 4mA. Following is the schematic of the board provided by MikroElektronika.
Image adapted from: MikroElektronika MIKROE 4-20 mA T Click schematic
I have written a program (via SPI) to both loop through the voltage range of the MCP4921 (with values from 0-4095 and back down again), and set the voltage to a "static" 0V to test what the loop current is. But after I connect a 24V source on the screw terminals, run the script and measure the current through the loop, the current won't go lower than 4.4mA in either of the two cases.
I have confirmed that the voltage output of the MCP4921 uses the entire range from 0-4.095V (and back down again) when looping through its range, and 0V when setting the value to a constant low. And the voltage was measured by connecting Vout and GND_iso to a multimeter.
I have tried asking at a TI forum but they claim, to my understanding, that given the current circuit, it should even control a current range with its lowest value being 0mA. And that for it to start at a 4mA lower range a 102.4kOhm resistor should be connected between Iin and Vref on the XTR116. So I have no idea why it won't reach down to the lower current range, since the MikroElektronika board isn't equipped with the 102.4kOhm resistor in the first place.
Additionally, when looping through the DAC's voltage range, the transmitter has this weird behaviour where it slows down significantly when it gets to around 4.4mA and "speeds" up again after it has gotten to a slightly higher current.
Anyone have any experiences with this board that can tell me if I did anything fundamentally wrong or if there may perhaps be something wrong with the provided PCB or anything else?
If it helps I've programmed the SPI using an arduino uno.
For further clarification I have drawn how I connect the voltage source and current measurement for the MikroElektronika PCB. Which is as indicated in the figure above with "V" representing the 24V source and "A" how I measure the current.
Regarding the different voltages present existing on the board, either 3V3 or 5V is chosen to be the Vcc depending on the position of a jumper and this 3V3 or 5V is provided from the female connector from for instance a MCU.
Vcc_iso is the voltage provided by the voltage regulator on the XTR116 IC which is powered by the loop voltage on V+ to step down the voltage to about 5V. This can be seen in the figure for the typical application of the XTR116 in the figure below.
Image source: Texas Instruments - XTR116 datasheet