Output of function generator is clamped positive

Discussion in 'The Projects Forum' started by fender7802, Dec 19, 2012.

  1. fender7802

    Thread Starter Member

    Apr 7, 2012

    I'm using the Elenco FG-500 function generator kit, and when looking at the output on a scope, the sine wave is clearly sitting about 3V above ground. When the output is taken with a DC voltmeter, I get about that. Anyone have any thoughts about this?

    The schematic is on page 3 of this PDF:


    My thoughts were that there must be some unwanted capacitance in the output circuit, and it was the case that C7 was backwards, but after switching it I still have the same problem.

  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Well looking at the diagram cant see any dc offset adjustments, ??
  3. DickCappels


    Aug 21, 2008
    The output is above ground because that's how the function generator works. The negative power supply of the chip and all outputs will be above ground. One "fix" would be to float your circuit's ground at -3 volts. Another way would to be capacitively couple the signal from pin 2 to a grounded resistor (220 uf and a 10 k resistor should be pretty good down to 1 Hz. Connect the positive side of the capacitor to pin2 of the IC and put a 100 ohm resistor in series with the output of the circuit or much better yet, add a buffer.
    Last edited: Dec 21, 2012
  4. fender7802

    Thread Starter Member

    Apr 7, 2012
    That is very interesting, thank you.