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DAQ Card is inducing noise on an external clock signal resulting in false triggering

I am using an optical encoder as an external clock source for analog measurements with a PCI 6036 card.  The optical encoder signal is "filtered" using a Schmitt Trigger circuit and proper shielding practices are used on the encoder cable.  I am using a recent version of Labview.
When the TTL square wave signal from the encoder is viewed on an oscilliscope (without the DAQ attached) it is a picture perfect square wave, nothing that would cause any problems.
When I then connect the encoder output to my PFI line (with or without the oscilliscope) I get false triggering due to intermittent (seemingly random...) high frequency noise "blips".  Out of 360 expected samples, I will typically get between 2 and 6 "bonus" samples...  When captured on a scope, the noise looks like a decaying sine wave and lasts for only a few us.  the peak magnitude is tyically around 2 V or so as shown on the scope, which is apparently just enough to make my card grab a sample.
Since the noise is only present when the encoder signal is attached to the DAQ, it seems that the DAQ is somehow inducing noise into the signal.
I have been fighting this problem for a while now and tried the following:
1)  I first tried the raw encoder signal, but then added the Schmitt trigger, increased the signal strength of the encoder lines by adding resistors to ground, double checked my shielding, etc...
2) I verified that my ground potential between my card and my conditioning circuit were not causing problems.  The conditioning circuit and encoder is now powered of the card itself, which should resolve any possible problem with grounds.
3) Cursed at various inanimate objects (made me feel better but didn't help the situation)
4) Checked if I could set a minimum pulse width required to trigger off of an exernal scan clock (I can't with my hardware.)
5) Swapped my card with a card of a different type (problem is still there)
If anybody out there has some recomendations, I am open to anything.
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Message 1 of 13

@OSU_Mech_Eng wrote:
I have been fighting this problem for a while now and tried the following:
1)  I first tried the raw encoder signal, but then added the Schmitt trigger, increased the signal strength of the encoder lines by adding resistors to ground, double checked my shielding, etc...
If anybody out there has some recomendations, I am open to anything.

is you schmit trigger out tied directly to PFI and the ground from ST directly to digital ground of your board?

I'm also curious about your 2V level, since the ST output FET will be smoked being low and pulled up with 2V  ...ah which ST do you use?


Greetings from Germany

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“ground” is a convenient fantasy

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Message 2 of 13
The schmidt trigger is a TI cd4093BE
As for the impact of the 2 V signal at the output of the ST, it is a very brief event.  This has probably has allowed it to survive sinking the current.  (I am an ME, so this is just a hunch...)
If I can not solve the problem directly, I am considering the following work-around:
Use a counter to do a retriggerable pulse generation.  Use my encoder signal as the gate and the internal clock as the source.  By specifying the count down values, I can specity the minimum pulse width required to trigger and the the resulting pulse width.
I haven't worked much with counters.  Does anybody have any comments on this approach?
Message 3 of 13
Your description of a decaying sinusoid at the transition sounds like a reflection from an improperly terminated cable. How long is the cable connecting the ST to the DAQ device input? Since the DAQ input responds to a signal that may a few nanoseconds long, any cable over a few meters probably should be treated as a transmission line, and will require appropriate termination, and care is selecting the cable itself.
John Weeks

WaveMetrics, Inc.
Phone (503) 620-3001
Fax (503) 620-6754
Message 4 of 13
My cable is a 1 ft long BNC cable from the output of the Schmidt trigger to the input of the card.  I have a BNC 2090 as an accesory to my card.  I have routed the PFI 2 line to one of the spare BNC's which I set as the clock source in my VI.
The noise is very random, only occuring once or twice every 100 or so periods of the clock square wave.  I would expect to get some ringing at sharp transitions of the clock signal, but the noise blips tend to happen towards the middle of the low phase of the clock signal.
Thanks for your input.
Message 5 of 13
I guess I misunderstood the exact nature of the noise. Your setup doesn't sound like it should have transmission-line troubles for sure.

I guess it's from something that doesn't happen very often. Any high-current switches in your system? Motors? I'm grasping at straws...
John Weeks

WaveMetrics, Inc.
Phone (503) 620-3001
Fax (503) 620-6754
Message 6 of 13

Pg 14 on this App Note may be describing your situation using a buffered chip. Do you have any of the unbuffered (UB) versions?

What were the results when you fed the encoder directly into the counter?

"It’s the questions that drive us.”
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Message 7 of 13


Your straw grasping is close to the mark.  The application used in an engine test cell with a 200 Hp DC motor as well as the ingition system of the gasoline engine.  My first guess was that this was the problem and I proceeded to shield everything and reroute cables to get some distance between my signal wires.  I'm 99% sure that this is not the noise source.

The strongest evidence (to me anyway) is that the problem does not show up when the DAQ is not connected.  I can scope the signal and it looks perfect, as soon as I hook up the line to the DAQ I can see the noise blips.

Thanks for the extra brain-power.

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Message 8 of 13
Thanks for the link, that does look similar to what I have.
My noise is slightly different, it typically occurs far from the transition from high to low or low to high.  It also goes above and below the signal ground, I'm not sure what this means...  It seems like the article describes a phenomenon where the output changes state a few times due to some AC noise before it finally changes state.  What do you think?
I will check and see what other cards we have laying around the lab.  If I have an unbuffered card I will stick it in and give it a whirl.
As for plugging the encoder straight into the DAQ, that is what I started with and still had the problem.  I then added the conditioning circuit (Schmitt Trigger) because I thought the noise was some type of induced noise problem from the surroundings.
Thanks for the suggestions!
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Message 9 of 13
Hello OSU_Mech_Eng,

I'm not quite sure how the DAQ card could be inducing glitches into your digital signal.  Digital signals from mechanical devices like quadrature encoders can often be glitchy or bouncy, but your schmitt trigger should act as a debouncing filter to clean up that digital signal.  It sounds like you have thoroughly troubleshooted this problem, and I would recommend moving on and trying to use a counter on your DAQ board to generate the digital signal, rathar than using the raw signal from your encoder/schmitt trigger.

You were correct when you stated that the best way to do this is by configuring your counter to perform retriggerable pulse generation.  You can use the signal of from your encoder to gate the internal clock on your DAQ card, creating a clean digital signal.  By setting the minimum pulse width of the signal, you will be able to ignore the small glitches in your signal. Here is a link to some Knowledgebases describing how to do this:

How Do I Remove Glitches or Add a Debounce Filter to My Digital Signal?

How Do I Define the Parameters for Pulse Generation in NI-DAQmx?

For further reference, on all of NI's new M-Series DAQ cards (PCI 625x), the PFI circuitry contains built in debouncing filters to protect against small glitches in digital signals.  If you have an M-Series card lying around, it might be helpful to give that a try.

I hope this helps,

Travis Gorkin
Applications Engineering
National Instruments
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Message 10 of 13