AD654 A/F nul output needed

Discussion in 'The Projects Forum' started by mikeocube, Jan 10, 2015.

  1. mikeocube

    Thread Starter New Member

    Aug 3, 2011
    Hi, I'm working on an analog to frequency converter based on AD654. I have a problem because the input signal (coming from a uP board D/A converter) always generate a minimum 5 mV DC which generate output pulses out of my AD654... This circuit is used to control a Step Motor drive and I should have a zero pulse output.

    First solution is to have a 10 mV offset circuit at the input. Second solution is a comparator which shot down the output when the input is under 5-10 mV (precision is not very important).

    Any other solution welcome.

    - I only have single supply available for this circuit +24V or +5V
    - No place for potentiometer on the board
    - I have 1/2 LM358 Opamp available

    I'm not familiar with these small DC analog signals. If you can help, I will appreciate.:) A_F_Conv.jpg

  2. DickCappels


    Aug 21, 2008
    Your comparator idea is the best of the two and probably better than I could come up with.

    On the the LM358, the + input connected to AD654 Vin, -input connected to a reference voltage, perhaps 10 or 15 mv -whatever works, but it would probably be higher than 6mv to assure that the input offset voltages don't prevent the circuit from working. A small signal diode in series (cathode on the LM358 output, anode to AD654 Fout) will shut down the output per your solution.
    mikeocube likes this.
  3. mikeocube

    Thread Starter New Member

    Aug 3, 2011
    Diode at the output of the 358 is a good idea. I will check for a stable reference IC and also components with comparator and reference in the same chip like Max 9016, LT6650, Max9062, Max9063, TLV3011.