XTR115: The output issue for XTR115

Hi Mickey,
Our XTR expert is currently out of office, but looking at this circuit, I don't see any obvious issues. The input voltage range will depend on the value selected for R56 and R57. With an 18k input resistor, 3V at Vout becomes a 166µA input current, which would translate to a 16mA output current.

What is the customer's output current range? Is this a standard 4-20mA output, or a different range? When output current is measured during correct operation versus the fault mode? If the customer swaps a faulty XTR115 back into the circuit, is the output correct for a short time still, or is the failure permanent?

Hi Tim,

OK. Thanks for your help.

Q1. If the customer uses 3-wire application, such as XTR111, I would like to get how to design the schematic. For the Figure 36 of XTR111, such as the attach, are the two red circles GND connected together?

Q2. Is there any blog about 3-wire application, such as XTR111?

Q3. Regarding the figure 3 of the attached article that you has sent to me, it shows "

The 3-wire sensor transmitter topology (Figure 3) supports sensor transmitter designs that require >4 mA of
operating current." Would you explain why the sensor transmitter designs can be higher that 4 mA?

Hi Mickey,

For Q1: Yes the ground connections are intended to be the same because in a 3-wire design the general assumption is that you have a power supply local to your transmitter and thus run a ground wire out to your sensor. Depending on the wire length however, your sensor ground may be at a slightly different potential. As long as it is not significantly different and you remain within the compliance limitations of the XTR111 then there is no issue.

Q2: The DAC group has a blog series about 3-wire industrial applications that you can find here: e2e.ti.com/.../industrial-dacs-an-overview-of-analog-outputs-and-architectures

Q3: For 2-wire devices, all of the current consumed by the input circuitry is returned through IRET and becomes a part of your output current. Consequently in a 2-wire system you can't have an output current less than what your input circuitry consumes. In a 3-wire application, the input circuitry is referenced to ground and it's current consumption is unrelated to the output current, so you can consume as much current on the input stage as you need.