VRef on LM4040 dropping when connected to eTape level sensor.

Discussion in 'General Electronics Chat' started by systemlordanubis, Nov 10, 2015.

  1. systemlordanubis

    Thread Starter New Member

    Jul 30, 2015
    17
    0
    Hi All,

    I have a circuit in which I'm using the Milone eTape liquid level sensor ( http://milonetech.com/products/chemical-etape-assembly ) with an output as a voltage divider. For it's V+, I'm wanting to use the 4.096VRef of the LM4040AIM3-4.1/NOPB. A circuit diagram is attached.

    However, whenever I connect the eTape, the VRef drops to around 3.9v (depending on the resistance of the eTape).

    The LM4040 has a current range of up to 15mA and from my calculations, the eTape should only draw up to 1.8mA, so the VRef shouldn't be getting over driven, but I can't work out what's going on.

    I need to get a constant and stable VRef into the eTape so that it's readings are relevant.

    Any thoughts would be appreciated.

    Thanks.
    Anubis.
     
  2. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    See

    REF.png
     
  3. systemlordanubis

    Thread Starter New Member

    Jul 30, 2015
    17
    0
    Hi Bordodynov,

    Thank you for the reply.

    The VRef part of the circuit (750ohm and 10Kohm resistors) were based on this circuit on Adafruit ( https://www.adafruit.com/products/2200 ). As far as I understood it, these resistors brought the voltage close to 4.096 (in testing, around 4.65v) and the shunt corrected it to 4.096v. A deeper look at the LM4040 data sheet seems to suggest that maybe the 10K isn't required?

    Thanks.
    Anubis.
     
  4. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    Yes, the 10k resistor is not needed.
     
    systemlordanubis likes this.
  5. systemlordanubis

    Thread Starter New Member

    Jul 30, 2015
    17
    0
    Hi Bordodynov,

    You're absolutely correct, removing this resistor gives the desired result!

    Thanks so much for your assistance.

    Anubis.
     
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