SCXI 1530 filter frequency and scan rate

[oly]

Hi,

I have couple of questions about SCXI 1530. Could you help me understand them？ Thanks. I found this from the user manual of 1530.

The minimum scan rate for the module must be at least 10 times the highest filter bandwidth setting.

1. Could you let me know why?

2. SCXI 1530 has 4 settings for filter bandwidth, 2.5, 5, 10, 20 kHz. If I am only use one channel and set the filter bandwidth to 20KHz, then the minimum scan rate should be 200 KHz (10 times 20KHz) based on the above information. 

However, in the manual "The maximum scan rate for the SCXI-1530/1531 module in multiplexed mode is determined by the formula: SMAX = 1/(8 μs+(N– 1) 3 μs), where N is the number of channels to be scanned."

The maximum value of the formula above is 125,000 Hz when N=1.

Does this mean 125,000 Hz is the maximum scan rate of 1530? If so,  12.5 KHz can not meet the 200KHz minimum scan rate requirement when using filter bandwidth 20KHz. Does this mean 20KHz setting can never be used?

3. Gain setting for 1530.

"This amplifier boosts the incoming signal by a gain of 1, 10, or 100." Will this change the output value? For example, the input voltage is 0.4mv, will I get output as 0.4mv, 4mv, 40mv respectively if I am using gain setting 1, 10 and 100? Another thing I am wondering is that if the input voltage is 2V, if I am using gain factor 10 and 100, what would happen?

4. Does input limits setting affect gain settings?

Thank you,

[Margaret F.]

Hello,

Thank you for posting on the NI Forums!

1. You are correct that the min scan rate must be 10 times the highest filter bandwidth setting. I believe this is specified so as to prevent aliasing. Typically sampling at 10 times the frequency of the signal of interest will yield the full details of that signal.

2. You are also correct that the max scan rate is 125kHz, however this is for parallel mode with sample and hold enabled. If you look at the top of page 30 of the manual, you will notice that there is no limit for parallel mode with sample and hold disabled.

3.  Page 39 of the manual dicates that 42V is the max voltage the card can withstand without being damaged. Therefore you should not amplify any voltage signal above this limit.

4. The input limit setting will not affect the gain. The input limit simply ensures that the entire resolution of the ADC is spread accross an input range that is more applicable to your signal. The gain can be set according to page 19 of the manual.

Hope this helps!

[oly]

Thank you for your reply!  

For question 2, 

For multiplexed mode and parallel mode with sample and hold, the maximum scan rate, 12.5 KHz, can not meet the 200KHz minimum scan rate requirement when using filter bandwidth 20KHz. Does this mean 20KHz setting can never be used for these two modes? 

Question 3, 

42V is the max voltage the card can withstand without being damaged. Therefore you should not amplify any voltage signal above this limit.

I am confused by this. Coud you help me understand? Do you mean input range of this module is 42V/Gain?

Input range    Gain

  +- 42V          1

  +- 4.2V        10

  +- 0.42V      100

From the following documentation, http://www.ni.com/pdf/products/us/4scxisc290_ETC_196.pdf

On page 3, we can see the maximum input range is +-10V.

The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the measurement system ground. 

Based my understanding, they are two different things. 

Input range is +-10V means the vlotage difference between two input terminals should stay within +-10V.

common-mode voltage range is +-24V (maximum +-42V) means voltage difference between each input to system groun should not go beyond this. 

Also for amplifier output range which is limited to power supply voltage due to saturation. Based my understanding, evne if the input is 0.3V with gain setting 100, the output voltage will be the supply voltage instead of 30V. Am I right? Btw, what is the supply voltage for SCXI 1530 normally,10V? 

[Margaret F.]

Hello,

Your last few questions will take some further investigation on my part. I will ask R&D and post back.

Have a great day!

[Margaret F.]

Hello!

In regards to your first question, the only way to achieve the max samping rate for the card is to use parallel mode without sample and hold.

For your second questions, please note that in the specs the operation common-mode voltage is +/- 24V, so you should stay within this range. 42V is the absolute maximum before damage occurs.

For your third question, the output range will be equivalent to your input range. However the signal is amplified between input and output, it will be limited in software to be outputed within the input range you specified.

Hope this helps!

[oly]

Thank you.

I guess I am still uncertain about the question 3. 

So as a signal conditional card SCXI 1530 did not change its output range? Say if the input is +-0.5mv, the output will still be within +-0.5mv regarless of whatever gain settings I am using. 

So the whole procedure is +-0.5mv signal is send to input, and the signal will be amplified by G(gain, say 10), the output range of the amplifer will be +-5V, which will go through low pass filter, sampling. After sampling, the data will be divide by G(10) back to range +-0.5mv?

Let's say, if i am connecting this signal from scxi 1530 to DAQ card,  then the actual DAQ card input range is +-0.5mv? Is the DAQ card reading the digital signal then?

 Thank you.

Message Edited by oly on 02-02-2010 05:20 PM

[Margaret F.]

Hello!

Thanks for posting back! I think I can answer your overall question by saying that  whatever input range is set in software for the 1530 will be handled by the driver to ensure that the output is within the +/-10V output range of the card. This ensures that the DAQ card will not be damaged. 

Please let me know if you have any further questions!

Thanks!

[oly]

Hi Margaret,

Thanks for your reply. 

What I would like to know is if the signal will be amplified by gain at the output end . From your last post, my understanding is No. Am I right? 

The signal will be amplified by gain for sampling to get better resolution, however, after that, the signal will be scaled back to the original range.

Thanks.

[Margaret F.]

Your last statement is correct. The amplified signal will not be the output of the card. The gain applied will serve to make better use of the resolution of the card but the voltage will be scaled back down for output.

Hope this helps!