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trying to use Picolog with an ADC 216

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trying to use Picolog with an ADC 216

Postby P.F. R » Wed Apr 14, 2004 8:17 am

Hi,

I'm trying to record some acoustic signals using an ADC 216 and Picolog in order to analyse it with a home-made Matlab routine. I've tried to set the sample frequency at 1 kHz, but it doesn't seem to be the real sample frequency...
How can I know for sure the sampling rate with picolog? (I've made some tests with a frequency generator and the "power spectrum density" function of Matlab that shows the 1kHz freq. given in picolog is not the real sampling rate)...
:?: help is all I need !
thanks alot
P.F. R
 

precision

Postby P.F. R » Wed Apr 14, 2004 9:02 am

I forgot to precise that I was using the "real time" of picolog.
P.F. R
 

Postby markspencer » Thu Apr 15, 2004 8:13 am

Hi,

I am sorry to hear that you are experiencing this problem, when Picolog is using real time continuous mode it uses the Windows clock for its timing. This means that it will not always get the data every millisecond, also to measure a 1 kHz signal you need to sample faster then 1 millisecond.

A solution would be to use 'Fast Block' mode and set the timebase and oversample to achieve the sample rate you require, to measure a 1kHz signal I would recommend sampling at least 8 times the frequency.

Best regards,
Mark S.
Regards,

Mark Spencer
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Postby Guest » Thu Apr 15, 2004 3:35 pm

thank you for your reply.

The problem I have with 'Fast Block' mode is that the recordings it makes are very short (and I'm planning to record for a few hours). This means that I'll have hundreds of 50 sec (for instance) long files. Is there anyway to make the recording in one file -even if it's not continuous (due to the time used my the ADC to empty its memory)?
I'm interested in lower than 1kHz frequencies (around 100 Hz). I made the tests I told you about at 100 Hz (and not 1 kHz as I wrote earlier).
Thank you very much.
Pierre-François Roux
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Postby markspencer » Fri Apr 16, 2004 7:52 am

Hi,

Since the frequency is only 100Hz, then at 1 sample a second this should be manageable, but as I said previously the unit gets its timing from the Windows closk, this is not accurate or the process or can take time out to do other things, thus stopping you getting a reading every ms.

However, this would be achievable if you use the USB to parallel port adapter. The timing for each sample will then be clocked by the USB device and not the windows clock. Therefore getting your sample every millisecond. The USB to parallel port adapter, has to be the one Produced by Pico, this is the only device that works with our products.

The price and description is available here:

http://www.picotech.com/usb_parallel_port.html.

When using the above product to measure 100Hz then real time continuous can be used, allowing 1,000,000 samples to be measured and saved in a single file. This will give you approx 16 minutes of sampling time.

Best regards,
Regards,

Mark Spencer
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Postby P.F.R. » Fri Apr 16, 2004 8:32 am

Hi,

the thing is that I am already using a USB to parallel port adaptater... and it's not working as you mentioned.
where is the problem?
P. F. R.
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Postby markspencer » Mon Apr 19, 2004 8:10 am

Hi,

I am sorry that I gave you incorrect information, I have tried it here and spoken to the development team to find out what is happening. Unfprtunately, the ADC-216, cannot collect a sample every millisecond in real time continuous. This is due to the unit not being able to stream. The only way you will be able to obtain the 1ms time interval is with fast block mode, but you are restricted to 32,000 samples, if only using one channel.

The ADC-11/12 would be capable of achieving this since it does have the facility to stream with the USB to Parallel port device. The voltage range on this unit is 0 - 2.5 volts.

Best regards,
Regards,

Mark Spencer
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