how this chip works


Joined Dec 25, 2006
On a high level it is a 4-20 mA loop transmitter. It is analog rather than digital and the loop current is the signal. Since the D/S says that it is 'loop powered' I am guessing it is a 2-wire transmitter rather than a 3 or 4 wire transmitter. But later in the D/S it shows an option for a 3 wire configuration, so both are possible.

Essentially you are taking an input voltage, from an RTD for example, and that is proportional to a 4-20mA current which is 'converted' back to a voltage across the reciever's load impedance.

There are many options with this one. Looks like you can control the output span, and offset adjustment as well.

One important note is that the input is usually floating in respect to the receiver ground. Looking at figure 15, the local power supply in the 3-wire configuration is referenced to COM rather than the receiver (load) ground. With the ADI part, the 'ground' is COM but is floating in reference to the receiver.

This also applies to the XTR115 from TI. The XTR115 has a pin called Iret where the input is referenced too. If this pin is grounded to the receivers ground, then the flow of current is incorrect and you will not see an output voltage across the load.

Thread Starter


Joined Oct 19, 2006
I really dont understand the Signal amplification stage. how do you know what voltage will be at pin 15 since there is a divider 800, 800 being fed back to the amp. I would expect the voltage at pin14 to be equal to voltage at pin 15. Is this true??
Also what is the major difference between a common mode ampplifier and and differential amp.
Common mode would mean that the inputs are reference to the chip ground and differential would mean that they are in rerefence to each other??


Joined Dec 25, 2006
I will take a harder look at it later, but it should be 6.2V because of the internal reference.
The internal reference is connected to the V+ pin, and the ouput of A1 will force V- to be equal to V+. Looks like a difference amplifier.


Joined Jan 17, 2007
TI had discontinued the XTR115. Which chips can be used as a replacement? I'm redesigning the board, so the replacement doesn't need to be pin-for-pin compaticble with XTR115.

- Nick

P.S. This a 6-month old thread, so I don't think I'm hijacking it.