SINAD challenge: Analog loopback distortion - how much for M series?

[Gustep]

Hello,

What is the best SINAD or THD that an M-series multifunction card can achieve when generating and reading back a 1kHz sine tone in a loopback test?

I seem to get variable results, with somewhere around 80dB at most when a direct loopback is used.

How many dBs should be possible? Can someone code an example VI that gets more than 80dB in a direct loopback and post it?

For example, a VI creates a sine wave at 1kHz and outputs this to AO-0. The AO-0 output is then looped back to a single analog input, AI-0. From AI-0, THD and SINAD are then determined with the "Distortion Measurement" VI.

Any sample rate and voltage range is acceptable - I am simply interested in what the best SINAD figure is that someone can achieve this way. It is probably best to generate a +/- 10V (peak) signal, and to configure the input for the same range.

Digital filtering or averaging should be kept to a minimum. A simply passive RC filter between AO-0 and AI-0 would be acceptable.


Once again, I am seeing only roughly 80dB SINAD at best. If someone can obtain a better SINAD figure with an M-series card, then please post the VI. Maybe there's a particular set of sampling rates and options that can be used to get a better SINAD?

Thanks!

[mallorim]

Hi Gustep,

Welcome to the forums! A few considerations when finding the SINAD of an M-series card.

1.  The M-Series cards do not have a specified THD or SINAD measurement.  NI does not test or guarantee any specific performance.  That being said, 80dB seems reasonable.  And your method for testing it sounds correct.

2.  For better SINAD or THD specs, I would definitely recommend a DSA (Dynamic Signal Acquisition) board.  These boards were designed with these specs in mind.

3.  For a cheaper but reasonable improvement another recommendation would be to use the Higher Accuracy 628x boards.  The THD and SINAD are different between our good, better, and best boards and you did not mention which card you are using.  One thing to note is that NI’s M-Series boards are not very phase accurate. It is likely if you want to know the THD and SINAD, you will care about phase information and should go with DSA.

I hope this provides you with some pointers. 

Mallori M.