• State Resolved
  • Locked Locked
  • Replies 12 replies
  • Subscribers 54 subscribers
  • Views 10133 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Single Supply 4-20mA Receiver with RCV420

Michael Mao
Prodigy 20 points
Other Parts Discussed in Thread: RCV420, OPA2237, OPA237, OPA188, OPA209, OPA277, TINA-TI, OPA333, OPA192, LM358

Hello,

I have an issue implementing a single supply 4-20mA receiver using the rcv420 chip. My design is based off the Application Bulletin of Single Supply 4-20mA Current Loop Receiver (http://www.ti.com/lit/an/sboa023/sboa023.pdf) and the rcv420 datasheet (http://www.ti.com/lit/ds/symlink/rcv420.pdf). My circuit is built according to figure 12 of the rcv420 datasheet, a 4-20mA to 5-0V conversion. The problem I am having occurs at around 16mA (or 1.25V output). From 4mA - 16 mA input, the output produces the correct voltage output range of 5V to 1.25V. But it saturates at 1.25V beyond 16mA, anything from 16mA to 20mA will only give me 1.25V output, but in theory 20mA should give me 0V output right? Does anyone know why this problem is occurring?

Thanks,

Michael

  • 0
    TI__Guru 62035 points

    Hi Michael,

    I know you've based your schematic from the one shown in SBOA023, but would you please show us your schematic displaying the actualy connections?  This will help us properly diagnose and solve the issue.

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Hi Collin,

    Thank you for the fast response! 

    I drew up a sketch of the connections used with the rcv420 and OPA2237 (suggested in the datasheet) here. I hope this helps in diagnosing the problem.

    Thanks,

    Michael

  • 0 in reply to Michael Mao
    TI__Guru 62035 points

    Hi Michael,

    I believe the issue is that the OPA237 does not have the output drive strength to enable this circuit to function properly.  I believe there may be a sentence in one of those app notes about the output current requirement of the OPA to perform the function needed.  I believe the short circuit current on the OPA237 is +4 mA / -10mA which will not sustain this application. 

    Please try the OPA209, OPA277, or OPA188.  Also if you'd like to play with a simulation to see how the circuit should behave please download the TINA-TI simulator:  http://www.ti.com/tool/tina-ti

    Then you can open the attached simulation file.  Please note that the simulation model for the RCV420 is idealized and the DC errors and output restrictions will not be modelled, only the basic transfer function. 
    2804.RCV420_Single_Supply+OPA209.TSC


  • 0 in reply to Collin Wells
    Prodigy 20 points

    I have +12V dc supply on my circuit.

    And I want to integrate your circuit to mine.

    Is +12 Vdc voltage enough to supply RCV420 and OPA209? Does circuit work accurately in linearly?

    Or do I need a +15V voltage ragulator in addition?

  • 0 in reply to Yigithan Eylik
    TI__Guru 62035 points

    Hello Yigithan,

    The minimum positive supply for the RCV420 is rated at +11.4V, so your +12V supply should be sufficient to operate this circuit.  Please let us know if you experience issues.

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Hello Mr. Collin

    Thank you for your comment.

    I am going to try the circuit..

    Then I will share my experiance...

  • 0 in reply to Collin Wells
    Prodigy 20 points
    Hi Collin,

    I am using rcv420 in my application where I have 12 V single supply and I want to convert 4-20mA to 0-5V to feed that to Micro-controller. Can I use the same circuit above.

    Ojas
  • 0 in reply to Ojas Kulkarni
    TI__Guru 62035 points

    Hello Ojas,

    As mentioned above, the minimum supply voltage for the RCV420 is 11.4V.  So the circuit should function as desired with a +12V supply.  If you test the circuit please let us know how it worked with the +12V single-supply connection.

    You could also consider a simple circuit with a non-inverting op-amp and shunt resistor as shown in the post below.  For a +12V supply, consider the OPA192 or OPA188 instead of the OPA333.

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Hi Collin,

    I tested the RCV420 circuit  with OPA209 with 12V single supply voltage but I am observing the same issue which Micheal observed. Output doesn't reduces below 1.25 V and remains saturated. Used connection diagram is below,

    Ojas

  • 0 in reply to Ojas Kulkarni
    Prodigy 20 points
    Hello,

    is there any update to this issue? I tested the opa209 and a lm358 and got the same saturation problems mentioned here. Does anyone have an op-amp rcv420 combination in single supply mode that works from 4-20ma and 5-0V?

    Thanks,

    Lars
  • 0 in reply to Lars Buhl
    TI__Guru 62035 points

    Hello Lars,

    After reviewing this thread from the beginning I'm pretty sure this design is not going to function as desired.  The creator of this thread referenced an application note about operating the RCV420 with a single-supply power-supply connection (SBOA023) and also Figure 12 of the product datasheet. 

    What I failed to notice the first time around is that the datasheet Figure 12 circuit requires a dual-power-supply (+/-15V) to properly operate.  Trying to achieve a zero-volt output with the RCV420 is not possible with a single-supply configuration due to the output swing limitations of the RCV420.  While the negative output swing is not directly defined in the product datasheet, the positive output swing limit is several volts below the power-supply voltage and the same should likely be expected on the negative output swing limits. 

    If we look back at the sboa023.pdf app note the single-supply circuit avoids the negative swing-to-rail limits by producing an output voltage of 10V - 5V which keeps the output at least 5V away from the negative supply, avoiding the swing-to-rail limits.

    As I frequently recommend on the forum I still think a single-supply op-amp with external shunt resistor is likely the best way to tackle a modern single-supply 4-20mA receiver.

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Hello Collin,

    first of all, i would like to thank you for taking your time to answer my question. I was hoping the RCV420 would be a great solution for me scaling 4-20ma current without an offset (in single supply mode). I'm now using a similar circuit you recommended before in this thread. This gives me the advantage of detecting an unplugged or broken cable. 

    Regards,
    Lars