Analog DC input buffers

Discussion in 'General Electronics Chat' started by steinar96, Feb 5, 2012.

  1. steinar96

    Thread Starter Active Member

    Apr 18, 2009
    239
    4
    Greetings.

    I've been trying to come up with a practical DC input buffer. I'm designing electronics around throttle pedals, the inputs are signals from potmeter configurations giving a DC input of 0.4-4.6 Volts. I'd like to buffer these voltages at the input. The application is a bit critical for safety reasons.

    I've ruled out bjt emitter follower due to the 0.7V drop.

    I've also run into troubles with a simple JFET source follower buffer. Both Altium and ltspice simulations show a varying output voltage depending on how the source resistor (3.3k-100k) is chosen (dc offset of 100mV to 1-2V in both directions around the input). Also a different choice of JFETs changes the output a lot in some cases. So practicalities/nonlinearities seem to be teasing me on that one, the circuit seems to be too dependent on the exact choice of components.
    http://socrates.berkeley.edu/~phylab...Files/bsc4.pdf (same JFET buffer as in this document)

    I may be wrong on this one, so please mention it if you don't agree. But i'm a bit afraid of unity gain op amps due to instability. Additional noise filtering (electric vehicles with 700A currents make for decent magnetic fields) may throw unity gain buffers into instability. And thus i haven't considered it a lot due to this concern.

    Anyone able to recommend something for me ?
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Use the op amp. Lots are stable at gain 1. If you do not like gain 1, attenuate the signal by 10 then aplify by 10. Protect ( as in filter, regulate.... ) the power supply to the op amp.
     
  3. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,795
    830
    Throttle for what? What type of electric vehicle? Is this an automotive application?
     
  4. steinar96

    Thread Starter Active Member

    Apr 18, 2009
    239
    4
    Formula Student class 1A, so yes. It's an electric race car.

    Throttle is designed by our suspension team. A mechanical construct with 2 potentiometer (for redundancy). My group (electronics) is building electronics around this.
     
    Last edited: Feb 5, 2012
  5. steinar96

    Thread Starter Active Member

    Apr 18, 2009
    239
    4
    [​IMG]
    This is what i'm working with at the moment for buffering the inputs. I've yet to test/simulate noise immunity though.
     
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