How do the limits in cal exec for vertical sensitivity of a 5911 relate to the vertical sensitivity spec (noise referred to input) in the 5911 user manual?

[Darrow]

One of our 5911's came back from calibration and it failed 'Vertical Sensitivity" at...

Range = 20V

Sample Rate = 2.5MS/s

Reading: 145.119 dBfs/sqrt(Hz)

High Limit: 152.000 dBfs/sqrt(Hz)

and...

Range = 0.2V

Sample Rate = 1MS/s

Reading: 113.859 dBfs/sqrt(Hz)

High Limit: 128 dBfs/sqrt(Hz)

How do these limits relate to the "Vertical Sensitivity" (Input Ranges) on page A-3 on the NI PCI-5911 User Manual?  http://www.ni.com/pdf/manuals/322150e.pdf  For 20V the user manual spec is 174 dBfs/sqrt(Hz) and for 0.2V is  the same 128 dBfs/sqrt(Hz).  

How might this impact previous error calculations?

Thanks,

Darrow

[Jennifer O]

Hi Darrow,

First I will link to the Manual Calibration Procedure and Specifications for this board for reference.

There are a couple of sources of noise on this module that need to be taken into account. One shown in the vertical sensitivity table which you have already referenced.  This is amplifier noise density which is dependent on gain settings.  The second is ADC noise density.  This is dependent on sampling rate.  These values can be found in the Dynamic Range section of the Specifications.

The two noise sources are uncorrelated, and depending on your combination of sample rate and range, one will typically dominate and can be used as an estimate.  Otherwise you can obtain the RMS total through the root sum of squares method.  This total is what is used in the calibration tests.

There is a VI that has been set up for calculating the noise-floor of the 5911.  You can find it here if you wish to see an illustration of the calculation (check out the block diagram).

If your module is failing calibration for any reason, you cannot rely on it meeting the accuracy specifications. This specific type of failure will impact the dynamic range/noise floor of the board.  Likely tens of dB that you have lost given the measurements you report.  Small signal measurements will definitely be affected.  I recommend repair or replacement to ensure the quality of future measurements.

Jennifer O.

[Darrow]

Hi Jennifer

It has already been repaired, I'm just trying to determine what impact this had on past measurements.  Your explination is very helpful...

Thanks!

Darrow

[Darrow]

Given the readings in my original post (elevated noise density at 0.2V range and low sample rates) and the gain noise and ADC noise specifications of the 5911, would it be reasonable to assume the source of the elevated noise density in my 5911 was probably a component in the amplifier circuit?

Thanks,

Darrow

[Jennifer O]

Hi Darrow,

I'm not sure how this question applies to the original one.  Are you trying to determine what was fixed on your board in order to have it pass calibration? Or do you plan to compensate your previous measurements depending on the source of the noise?  I want to help you move forward with your application, but I want to be sure that any answer I provide is relevant.

- Jennifer O.

[Darrow]

Hi Jennifer,

I'm evaluating the impact if any on measurements made prior to the repair.  The digitizer was only used in the +/-5V range at 12.5MS/s sample rate to measure a 2 square wave during that time  Given the noise specs of the 5911 relative to range and sample rate settings, compared to the call exec readings noted in my original post it would appear the source of the elevated noise was the amplifier.  The cal exec readings in the 20V range at high sample rates where ADC noise would dominate were were not effected.  I received a repair report from NI however it simply states a component was replaced with no other information.

Darrow

[Jennifer O]

To take a better look at your particular board I would like to know the serial number.  Perhaps it would be better to move this discussion to email to avoid posting any private information on the forums. 

You can reach me indirectly by contacting National Instruments Support.  If you refer to this forum they should be able to put us in contact.  Or if I have your permission I can obtain your email address from the NI Web team and contact you.

Thanks,

Jennifer O.
Product Support Engineer
High-Speed Digitizers

[Darrow]

Hi Jennifer,

Sure, you can contact me directly and you have my premission to obtain my e-mail address from the NI Web team...

Thanks,

Darrow