Probing my summing circuit

Discussion in 'General Electronics Chat' started by tomshong, Feb 27, 2016.

  1. tomshong

    Thread Starter Member

    Oct 6, 2011
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    0
    Hi experts,

    I built a standard summing circuit using LM318 following the schematic as shown. The only ‘load’ at Vout is the probe.

    My problem is, when I first power it on, the scope came up nothing, but when I disconnect and reconnect the probe, suddenly I start seeing the expect waveform and the circuit works.

    What could cause the problem?
     
    Last edited: Feb 27, 2016
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    Yeah, right a generic drawing of a summing junction. That tells us all about it.
    The probe is stopping the circuit from starting up.
     
  3. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    How would a probe from keeping the circuit from starting up? It's just a high impedance load, isn't it?
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    How would I know? Am I supposed to guess what kind of probe? What the power supply is? What frequency you are using? How you built the circuit?

    "I was checking a lever and it won't move. Why won't it move?"
    "How can checking it stop it from moving?"
     
  5. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    when i connect it up to the subsequent stage, the probe wasn't connected to anything, and nothing worked.

    The only way I can make it work is if i open up the connection between the output of the summing circuit and the following stage, then reconnect it, then it works.

    Attached is my summing circuit. It doesn't look much different than the generic schematic
     
    Last edited: Feb 27, 2016
  6. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    The power supply is +/- 9 volt, the two signals going in is 1) a sawtooth of about 25Mhz with about +/- 4Vpp and 2) a DC signal that varies between +/- 4volt using a pot. The summing circuit uses the DC to tune the offset of the sawtooth. I don't know the make and model of the probe. But at this point of my debugging I don't think it's the problem with the probe, since the same thing happens with the output connected to the subsequent stage without a probe, and the only way for it to work is if I manually sever the tie between the two stage then reconnect it before the summing circuit will work.

    I then isolated this summing circuit and only connect the output to the probe, and i was able to reproduce the same problem.

    It's as though the summing circuit wants to see an open circuit at the load first before seeing a load before it will turn on. I tried switching out the Opamp and it's still doing the same thing, so it's not an issue of faulty component.

    What could cause this phenomenon?
     
    Last edited: Feb 27, 2016
  7. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    Attached is the PCB layout on the summing portion of my circuits.
     
  8. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    The first thing I see is that the chip is good for 50 volts per microsecond with a +/- 15 volt supply and you are asking it for more than 628 volts per microsecond with a +/- 9 volt supply.
    The large signal output range is rated at zero by the time you get to 15 MHz.
    Definitely the wrong op-amp for this job.

    As a matter of experience, I say you should place a resistor on the output of the chip to isolate it from any capacitance on the load side.
    Still, you're lucky it works at all.
     
  9. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    Thanks. Looks like the voltage I was driving it was indeed the culprit. When I crank up the external DC supply up to at least +/- 12 it started working again.

    Looks like I will need to get a new onboard supply that gives it +/- 15 volts.

    Thanks again for your help! :)
     
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