Hi Eileen, If you perform your DAQ using the onboard timebase of your DAQ card, then the reading will be accurate, timewise, within the accuracy specified for the clock of the particular DAQ card you are using.
For example the card I use, NI-6070E, has a base clock accuracy o +/-0.01% (1/10,000th) (I looked this up online at ni.com in the spec sheet for the DAQ card). So if you are looking at the error on a time interval of 1ms then you would be off by at most +/-0.1uSec (a hundredth of a percent of a miilisecond). If you know t0 and the regular interval between each reading, there is no need for an explicit timestamp for each datum - you could calculate it, even to the year, and you would be as accurate on that as your PC can be at knowing the time of day and date (not very). But the intervals would still be within the spec for the DAQ card. So if you read the signal at 20MHz then each interval is 1/20,000,000Sec +/- 0.01% or 50nSec +/- 5pSec. It is simple muiltiplication to get this in terms of miliseconds (or DMY). There is no need for an explicit timestamp, just use the t0 of the PC for record keeping purposes and know that delta t is very good. Considering the time scale of an EKG the accuracy of most DAQ cards is many orders of magnitude finer. You wanted milisecond accuracy (1/1000th of a 60 BPM signal) and with 20MHz timebase you have milisecond accuracy +/- 1/10,000th of that. Is this making more sense now? Michael E. Ross Senior Design Engineer Standard Motor Products, Inc 2717 Commerce Road Wilson, NC 27893 Office: (252)234-5821 (voice mail) Lab: (252)234-2000 x4054 (no voice mail) Fax: (252)234-1900 [EMAIL PROTECTED] > -----Original Message----- > From: E.Mansfield [mailto:[EMAIL PROTECTED] > Sent: Wednesday, January 21, 2004 5:18 AM > To: Junginger, Johann; Info-LabVIEW > Subject: RE: Question Refined... > > > > Dear Johann, > > I agree with you about time stamps not all requiring the dd/mm/yyyy. I > am only a beginner at labVIEW and working on a very simple programme > (which I still can not get right) but all I need is > millisecond, second > and minute readings fixed to my waveform (a heart rate) not all the > other things. Is there not some way to get this on our data. > > Regards, > Eileen > > Eileen Mansfield > FELS / CHDL > 240D Briggs Building > Open University > Walton Hall > Milton Keynes > MK 7 6AA > > Tel: 01908 858824 > Fax: 01908 858868 > > email: [EMAIL PROTECTED] > > > > -----Original Message----- > From: Junginger, Johann [mailto:[EMAIL PROTECTED] > Sent: 20 January 2004 21:52 > To: '[EMAIL PROTECTED]'; Info-LabVIEW > Subject: RE: Question Refined... > > > > I have never been thrilled with the waveform data type, > although it does > make some things easier. Part of my gripe is the time-stamping: If I'm > acquiring 1.0 second of data at 10000 samples/sec, I do *not* want my > time values represented as D/M/Y H:M:S.xx. I suspect there may be ways > of customising that format, but they are never obvious when I'm coding > something. Why can't you easily force it to default to milliseconds? > > If I'm doing time based calculations (such as obtain the Y reading .25 > seconds after a trigger event has been detected, where trigger is on a > seperate analog channel) I find that forcing t0 to 0.0 (using a > constant) helps my numbers to come out right. > > Something else I usually do when I get tired of the conditions imposed > on me by the waveform type is to build a 2D array with X and Y as > columns (or as many Y columns as I have channels). You can > easily build > your X column in a loop using the dt property of the waveform (which > seems to default to seconds). This may be the solution for your Matlab > problem (I am not > familiar with Matlab syntax or data). > > It seems that even after the hysteria of Y2K time stamps and > time stamp > formats still cause people grief. I think a lot more LabVIEW > applications acquire data over a span of seconds rather than weeks or > days and I don't feel the waveform data type has been well implemented > in that respect. Those list readers who know better are invited to > correct me. :) > > Johann Junginger. > > > > -----Original Message----- > From: Thomas Gary [mailto:[EMAIL PROTECTED] > Sent: Tuesday, January 20, 2004 16:29 > To: Info-LabVIEW > Subject: Question Refined... > > > Wireworkers, > > With more investigation... I've found that my t0 variable on the Build > Waveform VI is a Time Stamp. I've never used a time stamp with my > academic Matlab code, so my next question is how do I convert a time > stamp into a Real number, namely zero. > > Gary L Thomas > >
