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TLV3691: Excessive input bias current

Rob Modeste
Prodigy 60 points
Part Number: TLV3691

Please disregard the previous post on this issue. I pasted in the incorrect circuit. This post has the correct circuit. 

I have a simple circuit where we are normally pulling the - input to supply. (1.8V) through a 1.5Meg resistor. The circuit is used to detect the connection of a battery module.

The - input of the comparator pulls enough current to bring the input voltage down to 1.65V. The input bias current should only pull 20pA so there should only be a minimal voltage drop. 

The device is rail to rail input so I don't know what is causing the excessive current draw. Any light you can shed on this is greatly appreciated. 

Below is the circuit schematic.

       

  • 0
    TI__Guru 52246 points

    Hi Rob,

    Ah..that makes more sense!

    You are correct that the TLV3691 input current should be in the pA's.

    What are you using to measure the voltage on SYS_DET? What DMM model?

    If you use a common DMM or scope probe with a 10Megohm input resistance, you create a 1.5M+10M voltage divider with R380 and the meter input resistance.

    With a 1.8V supply, adding 10Meg from SYS_DET to ground should generate 1.565V on SYS_DET.

    You will need a Gig-ohm input meter to measure correctly. Many higher-end bench meters can be switched into High-Z (Gig-ohm) mode.

  • 0 in reply to Paul Grohe
    Prodigy 60 points

    That makes a ton of sense. If I take that same measurement and reference to the 1.8V supply instead of ground then I should see near 0V. If there is real current flow to the comparator then I will see a similar voltage drop of about .15V.

    I will give that a try when I get to the office after the Memorial Day weekend and let you know what happens.

  • 0 in reply to Rob Modeste
    TI__Genius 11701 points

    Thanks Rob, glad Paul was able to help you. We’ll wait for your update

  • +1 in reply to Chi Nguyen1
    Prodigy 60 points

    Thanks for insight. Problem solved. This was just as you diagnosed. A measurement issue. 

    We don't have a giga ohm input impedance voltmeter so just referenced the voltmeter to the supply. We saw less than a 1mv voltage drop so the current draw from the input bias is pico amps as expected. 

  • 0 in reply to Rob Modeste
    TI__Guru 52246 points

    Hi Rob,

    Great! Glad we could help!