Oscillator changes voltage when connected to circuit

Discussion in 'The Projects Forum' started by mattswk, Aug 18, 2014.

  1. mattswk

    Thread Starter New Member

    Jul 22, 2014

    I have attached a picture of my schematic. I am having a problem with the frequency oscillator (the LTC6995). Unconnected at the "Freq" connection, the output of the oscillator is 1 volt. Once I connect it to the rest of the circuit, I get a 0 to negative 10 voltage. I want it to stay at 1 volt. Anyone know possible reasons for this unwanted behavior?

  2. Papabravo


    Feb 24, 2006
    I can barely read the schematic. It looks like the LT1228 has a bipolar supply and no GND reference. Is this correct? What makes you think you can connect things like this arbitrarily? Direct coupling ONLY works with circuits running off of the same supply with a common ground. What you are doing is a recipe for disaster.

    While you are at it try telling us what you are trying to do, then tell us if you can see any meaningful detail in the image you posted, and lastly find a way to give us a scaleable image we can actually SEE!
  3. to3metalcan


    Jul 20, 2014
    What's connected to your non-inverting input? What's the set-current for the device? What are your power supply rails? We don't have nearly enough info.

    I also don't really know what you're trying to describe happening. Is your signal turning into a square-wave? If so, it's probably excessive gain from the LT1228 driving it to the negative rail (acting as a comparator.)

    Also, I don't see that your LTC actually gets power? Both GND and V+ appear to be connected to ground.
  4. mattswk

    Thread Starter New Member

    Jul 22, 2014
    Sorry about the bad image. I have attached two images that make up the whole circuit. I spilt the cicuit in two so I could zoom in. I also attached the LT1228 schematic.

    The circuit has a thermophile at Vin (ST 150 from Dexter). The thermophile will detect heat and produce a signal. This signal will be compared with a frequency oscillator at a known voltage and frequency. Then the LT1228 will variable gain, less gain for large signals and more gain for small signals. The Iset is determined by the diffference between -10 volts at V8 and the incoming Vout below the LT1228 at LT1001. The output at O should produce a square wave that flucuates with the signal. Then it will go through another amplfier to be reference to -10 volts. Later on, the signal output will be compared with the frequency oscillator to determine how much gain the LT1228 produced at each point in the signal.

    1) There is no set current for Iset on LT1228. It will vary.

    2) Power is shown in the first image

    3) LTC gets power from V5 which is set at 5 volts

    4) The LT1228 has +12V and -12V coming from the same supply

    So my problem, again, is the LTC outputs a square wave a 1 volt and then when I connect it to the circuit, it shoots to 0 to -10 volts.

    Let me know if the images are not clear.