My LM6211 is stuck

Discussion in 'The Projects Forum' started by Gwelec, Jan 2, 2015.

  1. Gwelec

    Thread Starter New Member

    Jan 2, 2015
    Hi, I'm wondering if anyone might have some advice on the following problem:

    I've developed the circuit shown in the attached image (showing how the circuit should work), which is intended to take a variable resistance sensor (R1) and create a 0-10V signal from the entire resistive range (0-240 ohms). I'm using an LM6211 for the op amp and an MCP1804 to supply 10V to the circuit.

    The problem I'm having is that the output is fixed at 10V, regardless of the Vin+ voltage. There is also a consistent 10V at Vin-, although I'm not sure if that's because the output is stuck, or if the output is stuck because of the 10V at Vin-

    I've tried a couple of different values for the feedback resistors, thinking it may have been an impedance problem, but that didn't seem to help. I've also tried using two separate power supplies for each input, and adding a load to the output, still nothing.

    Any suggestions how to fix this, or explanations as to why this is happening would be welcome.


  2. wayneh


    Sep 9, 2010
    You're too close to the positive voltage rail. That's not a rail-to-rail op-amp.
  3. Gwelec

    Thread Starter New Member

    Jan 2, 2015
    Where did you read that? (the datasheet I'm working off of says it is rail-to-rail, I think it's the latest revision [revision C])
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    What is it you want the circuit to do?
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009

    The LM6211 is a wide bandwidth, low noise op amp with a wide supply voltage range and a low input bias current. The LM6211 operates with a single supply voltage of 5V to 24V, is unity gain stable, has a ground-sensing CMOS input stage, and offers rail-to-rail output swing.

    The LM6211 is designed to provide optimal performance in high voltage, low noise systems. The LM6211 has a unity gain bandwidth of 20 MHz and an input referred voltage noise density of 5.5 nV/√Hz at 10 kHz. The LM6211 achieves these specifications with a low supply current of only 1 mA. The LM6211 has a low input bias current of 2.3 pA, an output short circuit current of 25 mA and a slew rate of 5.6 V/us. The LM6211 also features a low common-mode input capacitance of 5.5 pF which makes it ideal for use in wide bandwidth and high gain circuits. The LM6211 is well suited for low noise applications that require an op amp with very low input bias currents and a large output voltage swing, like active loop-filters for wide-band PLLs. A low total harmonic distortion, 0.01% at 1 kHz with loads as high as 600Ω, also makes the LM6211 ideal for high fidelity audio and microphone amplifiers.

    The LM6211 is available in the small SOT-23 package, allowing the user to implement ultra-small and cost effective board layouts.


    • (Typical 24V Supply Unless Otherwise Noted)
    • Supply Voltage Range 5V to 24V
    • Input Referred Voltage Noise 5.5 nV/√Hz
    • Unity Gain Bandwidth 20 MHz
    • 1/f Corner Frequency 400 Hz
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    The part is not specified at 10 volt supply. At 5 volts, it can push a 2k ohm load to within 50 mV of positive rail but at 24 volts it can push to only 400 mV of positive rail. Note that 10k ohm loads can be pushed closer to supply rail than a 2 k ohm load. That being the case, you have only 1 k ohm load.

    Fact 1. You are operating close to a supply rail.
    Fact 2. You are having trouble with your op amp.

    Conclusion, find another way to achieve your goal or find another op amp that does not have questionable performance at situation you are creating.
  7. LDC3

    Active Member

    Apr 27, 2013
    The 33 ohm resistor is too small and with the output of the Op-Amp is creating a voltage divider, which will give you 9.85V.

    Since your input is 0 to 10V, then I suggest you make a voltage follower. Just remove the 33 ohm resistor and replace the 1K resistor with a wire.
  8. wayneh


    Sep 9, 2010
    I think the "rail-to-rail output swing" wording is misleading. The inputs cannot be so near the rail. The TI data sheet says the common mode voltage range goes up to only 21.5V on a 24V supply, and just 3.3V on a 5V supply. I reasoned in #2 that on the OP's 10V supply, you can't rely on accurate sensing above ~8V.
  9. Gwelec

    Thread Starter New Member

    Jan 2, 2015
    Just wanted to say thanks for all the input! I adjusted the input voltages to below 8V and it worked perfectly.
    GopherT and wayneh like this.