Selecting the Correct digitizer

Question

Hi
I need to perform Voltage measurements  (Vpeak to peak, Vrms, Vmax and Vmin) produced by the Back emf of an specific Brusless DC Permanent-magnet Motor at Speed of 7,000 Rpm's
based on the following considerations:
MOTOR DATA:
Number of Poles in Rotor = 10 (5 North and 5 South)
Number of Motor Phases = 3
1 mechanical turn = 1800 elctrical deg per Phase. 
Speed: 7000 rpm /60 = 116.66 rps.....then 1 rev at 116.66 rps occurr each 8.571 mSec.
Voltage = +/-10V
....am thinking to use a the NI PCI-6110 Digitizer...4 Ch; 12bit; 5MS/s/Ch (http://sine.ni.com/nips/cds/view/p/lang/en/nid/203​056).
Is this Card optimum for my application?
Is there are other Card (more cheap) capable to use in my applicaiton?
What other consideration do I need to take in a count in order to make a good card selection?
Please help me with any comment ...
Thanks for your attention

Answer 1

Hi delt,
The most important parameter while choosing a data acquisition card is the sample rate. As far as I understand from your post, the frequency of your signals will be around 116 Hz (supposing a cycle of your signal per revolution). Your sample frequency should be at least 10 times the frequency of your signal in order to recover the shape of the signal, so you should sample at least at 1.16 kS/s (which isn't that much considering that the card you suggested samples at 5 Ms/s). So I guess that this card is quite too much for what you need if the frequency of your signal is indeed 116 Hz. So you could use a Multifuncional M Series card like the PCI-6221: http://sine.ni.com/nips/cds/view/p/lang/en/nid/141​32.
Another thing you have to consider is that some cards appear as simultaneous, this cards has an ADC for each channel and are more expensive, their sample rate won't reduce if you use all the channels at the same time. Other cards only have one ADC and a multiplexer, so the maximum sampling frequency divides per the number of channels used. For example, if the sampling rate is 833 KS/s as in the PCI-6221 and you use 10 channels, the maximum sampling frequency you will be able to achieve is 83.3 KS/s.
Other thing you should consider is the isolation, therefore there are some cards designed to work under noisy conditions common in industry (which might be the case considering you are using motors), so you might want to take a look at this industrial cards, such as the PCI-6230: http://sine.ni.com/nips/cds/view/p/lang/en/nid/202​499
I hope this information is useful to you.
Have an excellent day.