sample clock adjust external trigger threshold

[cecinix]

I'm trying to use an non-TTL external trigger source (~8 kHz square wave from 0 to 1.4 V instead of from 0 to 5 V) as the clock for an analog waveform voltage output. Is there a way I can manually change the threshold used for the clock source so that I can get this to work?

I'm trying to avoid having to fix this in hardware, which I believe involves building a comparator circuit to generate a TTL trigger signal from my 0-1.4 V square wave trigger signal.

Alternatively, is there a way I can generate a TTL signal that is synced to my 0 to1.4 V ~8kHz square wave trigger signal using either of these NI cards: PCI-6115 or PCIe-6323?

Thanks!

[Gregory]

Hi Cecinix,

I'm not sure I understand your question, but if you have your signal in the form of a 1D array then you can just multiply by 5/1.4

[cecinix]

Hi Gregoryj,

Just to confirm, my 0 to 1.4 V square wave signal is not generated by my computer but from an external device not controlled by Labview. Also the VI's that I am working with come from the NI-DAQmx palette of VI's. 

I think that there would be too large of a trigger delay if I digitized my 0-1.4 V square wave signal, multiplied it by a number, and output the result into my Sample Clock source input. The Sample Clock that I'm referring to comes from the DAQmx Timing VI.

My understanding is that, by default, Sample Clock source signals are expected to be digital TTL signals (0 - 5 V square waves), which I think means that my 0-1.4 V square wave signal would not be sufficient. I'm wondering if there is a way to manually alter/reduce the threshold the Sample Clock is expecting for the source signal such that my 0-1.4 V square wave signal would trigger it properly.

[Gregory]

I see, that is much clearer now. Unfortunately I'm not very familiar with the DAQmx functions, hopefully someone else can comment.

[GiantDeathRobot · Accepted solution]

Cecinix, you are correct. The sample clock signals are specified to be TTL signals, which means the minimum high voltage thresholds on the PCI-6115 and PCIe-6323 are 2.2 V and 2.0 V respectively. The digital/PFI input thresholds are listed in the device specification sheets, so they are hardware defined. Unfortunately, since all of the digital inputs on the cards you mention expect TTL voltages, this is something you will have to fix in hardware. A comparator circuit could work, as could a pull-up transistor network. 

What is generating the square wave? Would it be viable to generate a TTL clock signal on your NI DAQ card, and export that signal to the rest of your system? Typically a digital system is fairly tolerant of a little extra voltage, so that might be easier than adding voltage conversion circuitry. 

Regards,

William R.

Applications Engineer

[cecinix]

Thanks William, that answers my question. In my system, the driver of a resonantly scanning mirror generates the square wave to be in sync with the angular position of that mirror. Since the frequency of that resonant scanner slightly drifts in an unknown way with time, any TTL clock signal that I generate on my NI DAQ card would not necessarily match that drift, which would cause problems for me. Thanks again, I'll just work on fixing this in hardware.