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Scope Triggers Without Reaching Trigger Voltage

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Scope Triggers Without Reaching Trigger Voltage

Postby popernack » Tue May 04, 2010 8:29 pm

Hi-

I am using a Pico ADC-212 oscilloscope to try to verify the existence of voltage "spikes" in our data collection system. We have a Keithley 2701 collecting data at regular intervals that will periodically report high voltages. I hooked up the oscilloscope to one of the channels the 2701 is measuring to see if the voltage really is what is being reported. I set up the scope for a one shot measurement and set the trigger to ~8V. The trigger was activated at the same time the collection system recorded a spike. However, the output from the scope showed that the voltage was never anywhere near 8V. How could the scope trigger when the does not cross the trigger level? I have screen shots that show the voltage trend after the trigger event, but I don't know how to attach them.

Thanks
popernack
 

Re: Scope Triggers Without Reaching Trigger Voltage

Postby PeterF » Wed May 12, 2010 9:41 am

Hi,
What speed is your ADC-212? Are you sampling fast enough to see a fast spike? It may be triggering on this spike but unable to display it with the present settings.
Regards,
PeterF.
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Re: Scope Triggers Without Reaching Trigger Voltage

Postby popernack » Mon May 17, 2010 5:35 pm

How can I tell the speed? I never changed it from the install, so it should be the default setting. Also, when these spikes occur, the voltage is usually high for >1s, so I don't think this is what is happening. I know this because the 2701 logs the voltages that it reads along with timestamps.

I don't know much about electronics, but, is it possible that noise can cause an erroneous reading on the A/D converter of the 2701 and cause a similar effect on the Scope?

The reason I suspect the A/D conversion is erroneous is that the 2701 samples several channels once every 10 seconds. When the spikes occur, it starts at one of the earlier channels and all the rest of the channels read high. Every channel that reads high is off by the same percentage. The fact that they are off by the same percentage seems to indicate the A/D conversion is erroneous.

Thanks
popernack
 


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