Part Number: XTR116
Hello, we created an 8 channel 4-20mA output/driver based off the XTR116. We are experiencing three problems:
1. The datasheet clearly states that the XTR116 has a VREF of 4.096 volts. The chips we received are correctly labeled as XTR116, but they are outputting 2.5 volts and not 4.096.
2. No matter what current we provide on the IIN pin, we get roughly 40mA on the current loop, it doesn't change. We have a digital-to-analog converter providing voltages/currents to the XTR116 to control this.
3. We are using the VREF off the channel 1's XTR116 for the digital-to-analog converters reference voltage. If we don't have anything hooked up to this current loop, the XTR116 no longer outputs a voltage on VREF... it becomes zero.... which causes all of the other 7 channels to fail.
Here is the circuit schematic showing the 8 channel 4-20mA drivers, the digital-to-analog converter for setting IIN, and the power supply. The idea is that any device that receives a 4-20mA signal can just hook up to 2 of the 4 terminals labeled as ISOLATED_POWER, and ISOLATED_GND. The power supply is already tied into the current loop:
Any help or advice on these 3 problems would be greatly appreciated, as we already have PCB's of these made, thanks!
Hi Matt, all the circuitry you connect to the input of XTR116 must be supplied by the XTR116 itself!!! See figure 1 of datasheet. If you want to connect something to the input of XTR116 which is powered by a different supply voltage, you must introduce a galvanic isolation as shown in figure 3 of datasheet. Only by this the XTR116 is able to generate the correct 4...20mA output current. Kai
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In reply to kai klaas69:
In reply to Matt Roybal:
Hi Matt, yes, everything you power on the input side of XTR116 must be supplied by the associated VREG pin. See again figure 1 and 3 and read the section "minimum scale-current" on page 6. Try to understand how A1 in figure 1 is working. Consider all the currents that are flowing. Kai
Hi Matt, no, you cannot connect all the signal grounds together! Pin 3 of XTR116 is no ground pin, but a current return pin. Again, consider how the currents are flowing in your scheme and the scheme of figure 1 of datasheet: In the scheme of figure 1 of datasheet the current into pin 2 is only flowing through R1. In your scheme this current is also flowing through R1, but then back through R2 and out of pin 3. That's totally different from the intended operation of XTR116. Kai
the first two pictures show the correct flow of currents through R1 and R2 of XTR116:
And this picture shows the improper current flow, when pin 3 is misused as signal ground pin:
Hi Kai, something I just realized... there is only 1x DAC (8 channel DAC), but there are 8x XTR116... how is the current path supposed to work for that? Originally we were talking about using the XTR116 5V regulator to power the DAC, then the DAC's output goes back to the XTR116... but I am guessing I cannot connect all of the XTR116 in parallel and then into the DAC?
In reply to Robin Maeder:
Hi Robin, keep in mind that A1 always tries to keep the voltage difference between pin 2 and pin3 to zero. So, pin 2 is a true current input. And the voltage drop across R1, between pin 2 and pin 4, is "mirrored" across R2. When you don't connect pin 3 and do not use VREG and VREF, the driving DAC output tries to find an alternative path for the return current back to signal ground of DAC. But this again disturbs the balance between the currents running to and from the XTR116 (pin 7 and pin 4). The only way to make the XTR116 work properly, if you want to connect something to the input of XTR116 which is powered by a different supply voltage, is to introduce a galvanic isolation as shown in figure 3 of datasheet. Kai
Ok I understand. So as Matt wrote above, even when the x channel DAC is isolated from SPI it is not possible to use multiple XTR115 in "parallel" controlled from 1 DAC, right?
Is there another component to achieve such a circuit with 1 DAC controlling x current loops?
Thanks for your help
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