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Degraded Signal: PXI-2501 as 1-wire 48x1 w/ TB-2605

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Dear Friends,

 

I have a PXI-2501 configured as 1-wire 48x1 w/ a TB-2605 terminal block.   My goal is to route analog square waves generated with a PXI-6259 to a bank of several LEDs.   Last week I posted a question concerning the wiring of the TB-2605, and wired my block in accordance with the jpg I received(attached below).   Unfortunately I am experiencing a degraded signal once routed through the switch... very dim LEDs as opposed to very bright if I connect directly to the PXI-6259 AO channel.   My initial thought is that there is a poor GND connection somewhere in my wiring.   However, before I start disconnecting wires and chasing that grimlin with the volt-meter, I would like some consensus from other members on the diagram below.   Am I wired up correctly?

 

Thank-you,

Zach

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Hi Zach,

 

I'll let other members post, but I think you're wired up correctly.

 

The PXI-2501 has around 2 kOhms of channel-to-COM path resistance.  This 2 kOhms of additional resistance in the circuit will decrease the amount of current flow.  To check if this is the issue, add 2 kOhms of additional resistance when connecting directly to the PXI-6259 AO channel.  If the LED dims in a similar manner, then you've reproduced the issue.  

 

As a side inquiry, what is the analog square wave frequency?  The PXI-2501 has a -3 dB bandwidth at 400 kHz and -10 dB at 1 MHz.  Keep in mind that signals with frequencies at or above the PXI-2501 bandwidth specifications will be attenuated. 

 

Let me know if you have any questions!

 

Chad Erickson

Switch Product Support Engineer
NI - USA

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Chad,

 

Thank-you for your post!   To eliminate the possibility of a bad GND, I rewired the block.   I have the analog input on PINs 61(+) and 27(+), and only one LED analog out on PINs 67(+) and 22(-).   I set the switch to "ch0" and oscilloscope the input and output signals side by side.   Interestingly, the voltage is the same.   However, the post switch signal is not as clean, and shows static.   Your theory sounds solid.   I'm currently reading my Ohm meter manual to figure out how to measure this and verify.   In the event this proves to be the problem, what strategies do I have to resolve it?   Ramp up my PXI-6259 AO by 2k Ohms?   Please let me know if this is falls outside the scope of this forum.

 

Thanks,

Zach

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Accepted by topic author super-neuron

Hi Zach,

 

What features of the PXI-2501 make it suitable for your application?  Have you considered using a different switch?  The PXI-2503 has the same 1-wire 48x1 topology and can be used with the TB-2605 terminal block.  It's spec'd path resistance is < 1 Ohm and bandwidth is > 10 MHz.  The PXI-2503 uses electromechanical relays, so this module has a finite relay lifetime and a slower cycle rate in comparison to the PXI-2501.

 

Perhaps the PXI-2503 would work better for your application?

 

Chad Erickson

Switch Product Support Engineer

NI - USA

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Chad,

 

I really appreciate your input.   I agree that I have purchased the wrong hardware for my application.   If I keep my PXI-2501s, I'll have to develop some novel way to step up the current to get the results I need... not gonna happen.   I already ordered four new PXI-2503s, and am processing the returns through my sales rep.   Thanks again for your sharp eye on this, otherwise I'd probably still be scratching my head!

 

Sincerely,

Zach

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Hi Zach,

 

I'm glad I could help.  You should be good to go.  Once you get your new hardware, let me know if you have any further questions!

 

Best regards,

 

Chad Erickson

Switch Product Support Engineer

NI - USA

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