Maximum safe DC voltage into PXIe-5122

[gregoryng]

Hello, my task right now is that I would like to measure the line noise on a 28 VDC power supply and characterize its frequency spectrum.  I have a VI to do this already.  Normally I would place a DC block on the line to physically protect the digitizer with a capacitor, but I don't have one with me.

Is it safe to connect this power supply output to the Ch0 input of the PXIe-5122 digitizer card? I've looked through the specifications ,and it seems like if I am sure that I'm in AC coupling mode, then it is.  But if I am in 50 ohm mode, it could exceed the max ratings of the card.

[JKarma]

Hello gregoryng,

According to the datasheet of the device the voltage range is not supported even with an input load of 1 M Ohm (Table 1. Full Scale (FS) Input Range and Programmable Vertical Offset).

In this case, my recommendation would be to reduce the input voltage (external conditioning circuit) without adding any (or low as possible) frequency domain noise or use a hardware more adequate for this input range i.e PXIe-5164.

I find value in the following list of oscilloscopes  and their characteristics.

Regarding the configuration of the input impedance please take into consideration this link 

I hope this helps you.

[gregoryng]

JKarma, thank you for your reply and consideration of my question.  Sorry for the delay in reply.

I would respectfully tend to disagree with your use of that table as a specification of the maximum rated inputs, as I read it as only referring it only refers to the programmable software settings of the ADC.

The section in table "Maximum input overload", would be where I would expect to find the information I am looking for.  The line item "

1 MΩ  |Peaks| ≤42 V" seems like it is saying that if the absolute value of the voltage peaks are less than 42 volts, that this is acceptable?  And therefore if the expected DC signal is at 28V, then an AC signal on top of this, with AC voltages with amplitude up to 14V, that this would be acceptable?

I would really appreciate if an NI applications engineer would confirm if I am interpreting this table the way that that NI intends?