回复： Re: 回复： Re: Before meauring small current, whether it is need to wait several time after switching it to DMM ?

[小木虫]

Hi, I want to use PXI 2535 to switch small current, e.g. 1-100nA, and then measure it using 4071+4022. We know that bouncing would occur when a relay transitions between open and close. But the relay in 2535 is FET, not the same as relays in 2532. Whether we still need to wait several ms to get a steady current? And if, how long it should be to wait (if 10nA is switched)?

[chericks1]

Hi 小木虫 ,

Thanks for asking.  Although it depends on your application use case, typically you will not need to wait several milliseconds to get a steady current.  The FET is spec'd to operate in a maximum of 16 microseconds, and has no debounce time.  A minimum settling time of 2 microseconds is enforced by the driver to ensure that the command has been sent to the hardware.  Additional time still may be required for signal propagation through cabling and the DMM.

Let me know if you have further questions!

Chad Erickson

Switch Product Support Engineer

NI - USA

[小木虫]

Hi,Chad Erickson:

      Thanks for your reply. I'd like to know that typically how long should be waited (in the software setting) if a low current (e.g.10nA) is switched using 2535 (do not consider other conditions)?

      In this way, if a voltage signal (-0.1~ -1V) is switched with 2535, how long do you think is a typical waiting time, or not need to set in control software? I think it should be shorter than that switching low current.

Best regards.

小木虫 

[chericks1]

Hi 小木虫,

I don't think additional settling time is required.  The default settling time for the NI PXI-2535 should be sufficient.  

Additional low current switching advice: Be concerned with triboelectric/piezoelectric effects, leakage current in the switch, burden voltage, contamination/humidity, etc.  Also, I recommend to "make-before-break" relay connections when switching current because it's best to always have current flowing rather than to interrupt it. 

If you still have concern, then I recommend trying how different amounts of settling affect the measurements.  Remember, the amount of settling required always depends on the load anyway.

Hope this helps!

Chad Erickson

Switch Product Support Engineer

NI - USA

[小木虫]

Hi, Chad Erickson,

       I have not used "make before break" mode in my test.  I could not understand "...when switching current because it's best to always have current flowing rather than to interrupt it". I think we have to connect the DMM to different channels to obtain currents between them. Then we have to interrupt a current flowing before measuring it with DMM. Could you please give me an example (including a scan list) about measuring multichannel current with this mode?

 Best regards

小木虫

[chericks1]

Hi 小木虫,

I posted an example here.  Let me know if you require an example program.

http://forums.ni.com/ni/board/message?board.id=320&message.id=1509

Thanks!

Chad Erickson

Switch Product Support Engineer

NI - USA