What Is The Resolution Of My NI-9227 At Highest Sample Rate

Issue Details

I am trying to understand if the NI-9227 module will suffer any loss in its resolution when used at its highest sample rate available.

I know that some C series modules have high-speed or high-resolution modes. I saw listed the NI-9226 and the NI-9228 on some documentation, and I know that there is some resolution lost in case you would like to acquire at a high-speed rate, but I am trying to understand if this would be the case with the NI-9227 as well.

If that is the case,

• What is the fastest sampling rate that does guarantee 24 bits?

• How many least significant bits are effectively lost at 50kS/s (higher sample rate)

• What is the effective resolution at slower rates?

Solution

1. We guarantee 24 bits at all sample rates. However, this is what we call the 'marketing' resolution. No 24-bit ADC in the market actually has 24 noise free bits. Check this: How to look at Delta Sigma ADC resolution?

2. So then, what is the effective number of bits at 50Ks/s? To calculate that first we need to figure out the Signal-to-Noise Ratio (SNR). To do this we get max input signal that the module can support and divide it by the rms noise. Then from there we can figure out the Effective Number of Bits (ENOB). You can search online, if you are curious as to why the ENOB formula is such. I will not go into it here, but in short it has something to do with the nature of analog to digital conversion itself. So, for the 9227, the ENOB is as below:

   

   Only 13.3 bits for a 24-bit product? Yes, the 9227 doesn't have the best noise performance.

3. Now at slower rates, the noise gets better and the ENOB improves.