XTR115: How to Isolate IRET pin

Hi Reza,

have you thought about using the XTR111? It is different compared to the XTR115 regarding the floating issue.

Kai

Hi Reza,

Enclosed is the 4-20mA design notes. As Kai suggested, please check out 3-wire 4-20mA transmitter IC. If you have questions, please let us know.

/cfs-file/__key/communityserver-discussions-components-files/14/0184.4_2D00_20mA_5F00_Basics.pdf

Best,

Raymond

Hi Reza,

For unknown reasons, I am unable to see your reply via E2E. Over the email,  you asked "Is there any way to do this by XTR115?"

If you convert single ended output from MCP4922 to differential input, then it connects to XTR115. But I think that it will be easier to get 3-wire 4-20 mA transmitter instead. 

Best,

Raymond

thank you very much dear kai and raymond for your valuable helps.

raymond, As you said I asked "is there any way to do it by XTR115".

Then I decided to delete that post, read the file that you suggested and then ask my question.

As you said 3-wire transmitter is best suited for the design but I couldn't find XTR111 in my country. I found XTR110 but it is much more expensive, so I prefer to use XTR115.

in order to make MCP4922 output to differential I have to connect VSS pin to Iret instead of ground, isn't it? (then spi communication will fail)

Hi Reza,

We have single-ended input to differential output amplifiers, such as THS4531, OPA862 etc.. You may use these to convert your input signal from DAC. 

If you are concerned about the cost,  you may make you own Voltage to current converter. Convert your input voltage to 1-5V and generate output current 4-20mA using op amp and current mirror circuit. If you use 250 Ohm Rset resistor, at 1V, input it will generate 4mA, at 5V, it will generate 20mA. You can use BJTs or mosfets to make your current mirror circuit, and use mirrored current to drive your load. If you are interested, I will make a simulation for you (let me know your available voltage rails). It will be as accurate as XTR111, if you are taken care of mirror circuits. 

Please let us know how you want to do it. 

Please also take a look at this thread that it may help you. 

https://e2e.ti.com/support/amplifiers/f/14/t/770353

Best,

Raymond

Thank you again and more dear Raymond

I will buy an XTR110 and build the circuit to do the job for now, and then try to build it using current mirror in order to reduce the price.

you really helped me.

thank you.

the voltage rails are 5 volts and 24 volts.

would you please show me how to do that using current mirror?

Hi Reza,

if the XTR111 is not deliverable where you live, you could do it as shown below. The simulation shows the situation for three different burdens, 500R, 50R and 0R.

VCC=16V is the minimum supply voltage to allow the driving of a 500R burden. R8 is crucial for stability. Don't omit it! C2 merely increases the stability for very low burdens.

reza_4_20ma.TSC

The phase margin of this circuit is excellent:

reza_4_20ma_1.TSC

Kai