current sensing opamp

Discussion in 'The Projects Forum' started by yassser, Dec 23, 2012.

  1. yassser

    Thread Starter Member

    Jul 25, 2011
    91
    0
    hello,

    I want to sense the current in an h-bridge module , I used op-07 because it has low offset voltage , I tested it on a breadboard in the non-inverting dc gain configuration with a potentiometer on the input and it worked fine.

    [​IMG]

    when I install it in the circuit , the output is always slightly less than Vcc , (14v with Vcc=15 , 16.5V with Vcc=18) , this is the circuit schematic , is there something wrong with it?

    [​IMG]

    opo7 datasheet : http://www.ti.com/lit/ds/symlink/op07c.pdf
     
    Last edited: Dec 23, 2012
  2. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    You mention Vcc. What is the op amp negative supply pin connected to?
     
  3. yassser

    Thread Starter Member

    Jul 25, 2011
    91
    0
    vcc- (pin 4) is connected to ground , but pins 1,8 are not connected (and i don't know what offset n1 and offset n2 mean)
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Look at the datasheet page 5, shows how to connect the offset null pins, these are used to make the output Zero at different input levels.

    Again if you READ the datasheet you will notice the maximum output is +/- 13Volts @ >10K load (page 4)
     
    Last edited: Dec 23, 2012
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    But the OP-07 opamp has inputs that MUST be at least 2V more positive than its vcc- (pin4). The circuit will work properly if you add a negative supply voltage to pin 4. I don`t know why your breadboard circuit worked.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    The null pins are to make the output zero, with zero input voltage, not to offset it for different input voltages. To do that properly you need to add an external input voltage of the appropriate polarity.
     
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