Other Parts Discussed in Thread: XTR105
I have an XTR105 application that I have been making on and off for many years. It involves monitoring a potentiometer and reporting the pot position via 4-20mA. Basically, I would just place the pot element in between one of the constant current outputs and the RC network that returns current to I-ret. The pot wiper goes to the V+in pin. But one of the oddball main pots we use is made in such a way that at the full counterclockwise the wiper is still on the resistance element and as such is not at zero Ohms. This would give a low Vin voltage even when I want a 4mA output. To fix this, I use the other constant current output through a trim pot to the same RC network feeding I-ret. So now when the pot is at full counterclockwise and maybe 15 Ohms, I can set the trim pot to 15 Ohms also and thus have the same small voltage on both the V+in and V-in pins of the chip, giving me a differential Vin of 0V and hence 4mA output. But I realize that if the trim pot is set to a higher resistance than the endpoint resistance of the main pot, the V-in pin will have more voltage than the V+in pin, so a negative Vin voltage. I don't see anywhere in the datasheet about what the XTR105 does with a negative Vin. Empirically, I have seen a number of units that have had such a condition and no apparent damage to the IC, and more importantly the I-out does not go below 4mA. However, I wonder if there is any other side effect of a negative Vin, such as increased heating, reduced accuracy, drift, reduced linearity, etc.
Anyone know the answer to this?
