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Inquiry about suitability of RCV420 to VDM28 laser distance sensor

Amit Kharat
Prodigy 20 points
Other Parts Discussed in Thread: OPA188, OPA209, OPA333, RCV420

Hello,

I am working on LASER distance sensor of pepperl+fuchs VDM 28 which gives output from 4mA to 20mA. Now currently i am converting this current into voltage using 250 ohm resistor and than it gives to 10-bit ADC of controller for actual processing. but my reading are very fluctuating so can i use RCV 420 current to voltage converter in order to increase accuracy ?? if it is than which schematic is useful for it ?. I am operating VDM 28 at 12V dc.

waiting for reply

Thanks, 

Amit

  • 0
    TI__Guru 62085 points

    Hello Amit,

    Most ADC's require an amplifier on the input to properly drive the sample/hold circuitry internal to the ADC.  Directly connecting a 250Ohm resistor to the input of the ADC may not provide a strong enough drive strength to properly charge the ADC sample cap to the proper voltage before the conversion begins. 

    We frequently recommend using a shunt resistor to GND and a rail-to-rail input OPA to gain / level shift the input signal (if desired).  An example of this is featured in the following forum post:

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/242351/850461.aspx#850461

    With a 250 Ohm load you can probably operate with a unity-gain buffer configuration because 20mA will produce +5V.  For +12V supply operation I would recommend using the OPA188 or OPA209 in place of the +5V OPA333 shown in the referenced post.

    Also, do you have a schematic you can share so we can review your basic connetions and circuitry?

     

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Hello Sir,

    I share schematic with you.

    I this VDM28 sensors output is 12V (4mA to 20mA).So in order to convert into voltage i use resistor of 250Ohm. but due to input current range is from 4mA to 20mA it gives output from 1V to 5V. I am giving this output to the ADC pin of micro controller Atmel 2560 which has 10-bit ADC which gives reading from 220 to 1023 steps. that means first 220 steps are of no use. In order to use all 1024 steps my out put is to be 0V to 5V witch leads to minimum step size and maximum accuracy.

    Now,

    I want to use RCV420 IC.The RCV420 is the only completely integrated solution for 4-20mA receivers.

    My question is, can i use IC as used in following schematic ?? if it is, then is it require any additional circuitry??

    and is it necessary to give negative voltage supply to pin no 4??? if it is please specify the range of negative voltage also.

    Waiting for your reply.

    Regards,

    Amit Kharat

  • 0 in reply to Amit Kharat
    TI__Guru 62085 points

    Hello Amit,

    In order to use the circuit in the configuration you've shown, you'll need at least a negative 5V supply connected to pin 4, as shown in the specification table of the RCV420.

    The RCV420 can be operated from a single +15V supply as described in the application report:  SBOA023

    However, in this configuration the output of the RCV420 ranges from +10V to +5V ( a -5V span) when referenced to the +10V REF output, the differential voltage will be 0V - (-5)V.  based on the information you've provided so far, this may or may not work for your application. 

    Let us know If none of the options shown in the SBOA023 app note will work for you and we'll work towards another solution for your application.

  • 0
    Prodigy 40 points
    Hi Amit,
    I am also got stuck with the same thing,
    Do u get any solution for this.
    I am also using VDM 28, is RCV 420 current to voltage converter is suitable for this.

    Thanks
    Amol
  • 0 in reply to Collin Wells
    Prodigy 40 points
    Hi Sir,
    I am also got stuck with the same thing,
    Do u get any solution for this.
    I am also using VDM 28, is RCV 420 current to voltage converter is suitable for this.

    Thanks
    Amol
  • 0 in reply to Amol Meshram
    TI__Guru 62085 points
    Hi Amol,

    I see that you've posted in a few other threads. I'll provide an answer in the post I just moved to the "Amplifiers and Other Linear" forum.