12-30-2013 01:26 PM
12-31-2013 01:53 PM
Hi Steve,
For DC measurements, generally you can use PMU function to force/measure voltage/current.
For timing measurements, how fast is your signal? if you set 6556 input to high impedance, will that destroy your signal integrity?
If 50ohm input impedance is indeed needed for signal integrity, you will get (50/1050)*3.3V = 157mV on 6556 side. you have the option to set VIH/VIL on 6556 with 25mV accuracy, so it will probably work if you set your VIL to 75mV or something, however it still depends on how accurate the 3.3V and 1Kohm are
Also keep in mind that the 1m VHDCI cable adds 100pF capacitance, so with 1K source impedance,delay will be introduced. It would help if you can buffer your signal before sending to 6556.
I hope this helps. Please let me know if you have any other questions
Qiong
09-23-2014 07:24 AM
Hi,
I'm trying to execute a similar test, but my pull-up resistor is about 50 kohms. I've been able to successly do HWC when setting VTT on the specific channel, but I was wondering if setting the channel to a different input impedance would help.
Do you mind answering the following?
I've read the manual but I haven't a clear answer for either. I think I'm missing something.
Thanks 🙂
09-24-2014 03:24 PM
Hi hch3,
The output impedance for the PXI 6556 is 50 Ohms. The input impedance can be selected between high impedance and a 50 Ohm impedance. Please reference the below document for information regarding the high impedance value.
What Does High Impedance mean in the PXIe 6555/6556 Specifications Manual?
http://digital.ni.com/public.nsf/allkb/07A48243A2A1AE2986257B7B0064AEC3
I am not familiar with what functions you will use to set the input impedance outside of LabVIEW. I do know that you will be able to use a property node in LabVIEW in order to change the impedance. What development enviornment are you using?
09-25-2014 11:42 AM
Hello,
From that same link above, in the Related Links section, is the KB for changing input impedance on the 6555/6556:
How Do I Set the Input Impedance On My PXIe-6556 to 50 Ohms?
To translate from LabVIEW to C (.dll), you will use NI-HSDIO Attributes NIHSDIO_ATTR_DATA_TERMINATION_MODE and NIHSDIO_ATTR_DATA_TERMINATION_VOLTAGE_LEVEL as indicated by the KB in LabVIEW. The C function to Get/Set Attributes should also be used with these attribute names to achieve the functionality you are looking for. We have our full C reference in our NI Digital Waveform Generator/Analyzer Help which can be found online in HTML format or on your computer as a .chm file.