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XTR111: Current Sink Vs Source

Part Number: XTR111
Other Parts Discussed in Thread: XTR110, XTR300, XTR305


We are using XTR111 as a voltage to current converter for a 4-20mA signal in the transmitter.  

Many times customers asked whether we provide current output as source or sink. 

I would like to understand the difference between both sink and source and how we can provide a solution if we want to give both the options in the transmitter. 



  • Hi Purushottam,

    The short answer to your question is that the XTR111 is not really designed to sink current, only to source it. It is a high-side voltage-to-current converter, where there is a loop current that is sourced and flows through the load resistance, allowing the voltage across the resistor to be measured relative to the loop GND.

    If you wanted to sink current, you'd need to essentially reference your load to the loop supply rather than to GND. You might think that you could just place the load between your Vloop (24V or whatever you choose) and the VSP pin of the device, and then just connect the output of the XTR (after the 15 ohm resistor) to GND, but that doesn't really work in practice.

    To show why, here is a mockup of the XTR111 with a 24V supply, 250 ohm load, and everything connected in a "normal" fashion to source current into the load. Note the 20mA across the load resistance at the bottom right.

    Now see what happens when you move the load resistor to the input. Note there is no longer 20mA across it, because the Rset draws an additional 2mA or so of current. This draw is not regulated the way the output current is, and again because this is just an idealized mockup and we don't have a real SPICE model for the device, we don't have any way to assure what the values would be.

    Note as well that any current drawn from your Vreg output will also be drawn through the VSP pin and will further impact the effective load current across your load resistor. Same thing with the Iq of the XTR itself.

    Now, the XTR110 datasheet does include an example circuit for a +/-200mA current pump (Figure 8, see page 9 of this document) that could theoretically be modified to achieve +/-20mA capability. However if all you need to do is sink 20mA, you'd save a lot of time and effort by simply implementing a discrete 4-20mA transmitter similar to the below concept (by John Caldwell).

    This post includes some other helpful resources regarding the design of a current sink for this sort of application - click me



  • Hi Purushottam,

    Also please note that the XTR305 and XTR300 are easily capable of +/-20mA current output, if bipolar supplies are available. This design includes two XTR300s, and walks through the component selection and protection considerations in detail.