Function generator problem

Discussion in 'General Electronics Chat' started by sudil99, Jul 19, 2012.

  1. sudil99

    Thread Starter New Member

    Mar 9, 2012
    15
    0
    Currently I am facing two problems with my function generator.
    1. analog.png In this attached cicuit if I give 60mV p-p from the function generator and I put small R3 like 2k, the oscilloscope reads almost zero signal (it has minimum resolution of 20mV). With 1Mega it reads like, 5mVpp. With 4.7k, it reads like 30mV p-p. If I remove R3, it reads correctly (60mV).

    2. Whenever I give input from the function generator to ADC or opamps, they show correct measurement only if oscilloscope is also connected to the input terminal (where the function generator is giving the input). If I do not connect the oscilloscope in input, then the output reads double of the expected value.

    Why is that?
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    What kind of function generator? One you built or a commercial unit? If commercial unit, what brand/model?
     
  3. sudil99

    Thread Starter New Member

    Mar 9, 2012
    15
    0
    Tektronix afg3022b
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    If you don't have the user manual, do a Google search on the model number and get one. The output impedance of the function generator is adjustable via user inputs. What do you have your's set for? (page 54 of User Quick Start Manual) It has a range of 1 ohm to 10K ohm.
     
  5. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    My experience with a sick output like that is that I shorted it out and burned up the output driver.
     
  6. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Typically, models such as these are rated into 50 ohms. If you do not load it with 50 ohms, then you get twice the output voltage ...which is okay, just reduce the output voltage setting to half of what you need. One way to get the proper voltage is to place the scope input channel to the 50 ohms setting.

    If this isn't working, then there is something wrong with the output driver of the generator.
     
  7. ramancini8

    Member

    Jul 18, 2012
    442
    118
    You say you put 60mv in, and with a 4.7K termination, you get 30mv out. If the numbers are true the output impedance of the generator is 4.7K. Having the scope connection change the output voltage means that the scope input impedance is low---about equal to the termination impedance according to the numbers given here..
     
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