ENB and jitter of NI PXI-4461 board?

[Ov_metas]

Hello!

I have 2 questions concerning the NI PXI-4461 board:

1) I wonder the equivalent number of bit (ENB) of the ADCs of NI 4461 board? I know that they have 24-bit sigma-delat converters but does the ENB depend on the signal frequency or/and sampling frequency?

2) What is the jitter that affect the sampling frequency ?

I tanks you in advance,

Best regards,

Frédéric

[sarahbl]

Hi Frédéric,

You can find the information about the Effective Number of Bits (ENOB) on this KB [broken link removed]. This KB is written for 449x devices, but those card are also DSA cards with delta-sigma ADCs. So you can use it for your PXI 4461.

You can find the Total Harmonic Distortion Plus Noise (THD+N) you need to calculate the ENOB on page 9 in the specs.

About your second question, I am not sure to understand what you are asking. Can you explain it please ?

Best regards,

Sarah BL

[Ov_metas]

Hi Sarah,

Many thanks for your answare!

Following the informations you gave me, the ENOB at a sampling frequency of 200 kS/s on the range gain of 10 dB (+-3.16 V) is only 17.85 bit !!! The dynamic range DR is 110 dB (see page 8 of the specs) and the formula is ENOB = (DR - 1.7609) / 6.0206.

I understand the DR=110 dB is only the "typical" value but the resulting ENOB of 17.8 bit is well below the nominal 24 bit of resolution!!

Concerning my second question: with a sampling frequency of fs=200 kS/s, the board will give me a value every dt=1/fs=5 us. The jitter I am looking for is the stability of this dt!

I hope that my second question is clearer?

I thanks you in advance!

Best regards,

Frédéric

[dsb@NI]

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Following the informations you gave me, the ENOB at a sampling frequency of 200 kS/s on the range gain of 10 dB (+-3.16 V) is only 17.85 bit !!! The dynamic range DR is 110 dB (see page 8 of the specs) and the formula is ENOB = (DR - 1.7609) / 6.0206.

I understand the DR=110 dB is only the "typical" value but the resulting ENOB of 17.8 bit is well below the nominal 24 bit of resolution!!

---

Yes, ~18 bit is much less than the nominal 24-bit resolution of the ADC, and this highlights the importance of good design of the analog front end AND setting the range of the channel appropriately for the signals you are trying to measure. 

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Concerning my second question: with a sampling frequency of fs=200 kS/s, the board will give me a value every dt=1/fs=5 us. The jitter I am looking for is the stability of this dt!

I hope that my second question is clearer?

Your question is clear. I don't see the clock jitter (or phase noise) specified in the 4461 module specifications though, and it is not a warranted spec verified during calibration. Out of curiosity, are you looking for the clock stability because you care about phase noise? Do you want to know what to expect, or would you prefer an experimental procedure to measure phase noise for your device?

[Ov_metas]

Thanks Doug for your answer!

I was looking for specification in the jetter because it is a parameter of the uncertainty calculation a of the phase of a measured sinewave.

[Henrik_Volkers]

If you want to measure phase relations, you usually capture two channels.

So you can measure the jitter of your setup by using one cable (you don't move 😉 ) between the two channels (providing a constant delay)  and do repeated measurements. (Or even better use a good power splitter and matched impedances on all input and output channels!)  

The phase error due to uncertain (and not matched) impedances of all elements involved in the signal chain will be higher (my guess)  

If you have a tightly locked system, a coherend excitation and capture of a 10 kHz sine,  a 3 parameter sine fit  will resolve  less than a cm length difference between RG58 cables (I got less than 10E-4 °  standard dev. :), uncertainty is higher mostly due to other components in my signal chain.. )

The noise is the limit and phase noise (jitter) is also only noise *, applying a sine fit is a narrowband filter with a bandwidth 1/{measurement time} , just like a FFT but without leakage (Actally, if the Fourier conditions are all met, it's the same) #

*) related to phase measurement