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TLV4112: TLV4112 input bias current

Abdelrhman Mohamed
Prodigy 40 points
Part Number: TLV4112
Other Parts Discussed in Thread: LMC6482

I am trying to use LTV4112 as a dual op-amp circuit to interface with an electrochemical sensor.

The operation depends on having high output current (max 360mA) but low input bias current.

I measured the input bias current to be 1-1.7nA at 350mA output current. However, the data sheet shows input bias at 0.2pA (typical) to 25pA (max).

Is this typical for the this part number? Or is it an indication that I am using this incorrectly? Also, if you're aware of any other op amps that can be used as a substitute, please let me know.

Attached is the circuit under discussion. The "dummy cell" is a replacement for the electrochemical cell for testing purposes. I tried adding 260ohm resistor which limited the current from 1.7nA to 1nA.

Thank you very much in advance.

 

  • 0
    TI__Guru* 93105 points

    Hi Abdelrhman,

    The TLV4112 is a CMOS power operational amplifier. CMOS transistors may have very low input bias current at 25 C and lower temperatures, but as the temperature increases their gate current goes up. The input bias current approximately doubles for each 10-degree increase in the temperature. Also, the inputs have ESD diodes to the power supply rails. Those diodes are off, but they still have leakage current and that increases too with temperature. In fact, this current is usually the major contributor to the input bias current. 

    Since the TLV4112 is a power op amp it heats up a lot when driving a high current load, and the input circuits get heated along with the output stage. This causes the input current to increase significantly over the quiescent condition, but this is normal.

    Any of the CMOS power op amps with exhibit this behavior, and bipolar power op amps have bias current that are typically much higher. You may want to employ a separate op amp stage that doesn't get heated in front of the TLV4112 to keep the bias current low.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points
    You're suggesting adding a voltage follower like (LMC6482?) where the 260 ohm resistor is?

    That sounds like it may work. I will try this and update this forum with the result.

    Thanks!