Cannot set full sampling rate when using 2 channels

[runson]

Hello,

I'm using PCI-6250 DAQ card and have been acquiring data using one channel at full sampling rate (1.25MS/s) with no problems.

Now, I want to sample two channels at the same time using 1 MS/s (which it should be able to do).  I've been trying to get this to work but with no luck.

The fastest rate I've been able to use without warning/errors is 500 kHz.  If I set the rate to 500 001 Hz then I get the following error:

--

Warning 200011 occurred at DAQmx Start Task.vi

Clock rate specified is so high that it violates the settling time requirements for the acquisition.

Reduce the clock rate, or the accuracy of the measurement might be compromised.

--

This error is displayed up to whenI set the rate at 625 001 Hz then in changes to

--

Warning 200012 occurred at DAQmx Start Task.vi

Clock rate specified exceeds the maximum conversion rate of the ADC. ADC overrun errors are likely.

--

This error is displayed up to when the rate is set to 769 231 then the error message changes to

--

Error -200019 occurred at DAQmx Read (Analog 1D Wfm NChan NSamp).vi

ADC conversion attempted before the prior conversion was complete.

Increase the period between ADC conversions. If you are using an external clock, check your signal for the presence of noise or glitches.

--

Attached is the VI that I've been trying to get to work (a modified example)

I'd really appreciate if someone can help me out here.

Best regards,

Runson

[runson]

Additional info,

I downloaded and ran the DAQ diagnostic Utility v1.0 and got the following results, see below

Regards,

runson

9/16/2005 6:54:58 PM

Results saved to:  C:\Documents and Settings\Runson\My Documents\Diagnostic Results.txt

Selected Device: Dev1

Device Type: PCI-6250

Serial Number: DE4115

Device Support: (PASS)

NI-DAQmx Version: 7.5 (PASS)

Device Reset: (PASS)

The device was last self-calibrated on: 9/15/2004 5:04:09 PM

Running Calibration ...   (PASS)

-------------------- Testing Counters... --------------------

Testing CTR Buffered Pulse Generation...   (PASS)

Testing CTR Single Pulse Generation...   (PASS)

-------------------- Testing Analog Input... --------------------

Testing AI Ground Reference...   (PASS)

Testing AI Voltage Reference...   (PASS)

Testing AI Finite Sample Clock...   (PASS)

Testing AI Finite Sample Clock Frequency...   (PASS)

-------------------- Testing Digital IO... --------------------

Testing DIO Lines...   (PASS)

Testing DIO Ports...   (PASS)

 ----- DIAGNOSTIC TEST COMPLETE -----

[WM_John_Weeks]

The specs for that device say 1.25 MS/s single-channel, or 1 MS/s multi-channel. That is not a simultaneous-sampling device, so you have to divide the rated sample rate by the number of channels you're sampling to get the max scan rate.

That is, the rated 1 MS/s is the conversion rate. To sample two channels it samples one channel, then the other. That process happens at up to 1 MS/s, so to get back to the first channel for the next scan requires another microsecond.

I guess the multiplexer they used for that board isn't fast enough to give good readings at 1.25 MS/s...

[runson]

Hello John,

Yes, I was beginning to arrive at that conclusion, thanks for verifying it.   This is very disappointing to say the least.

If I would add another 6250 card, can I be sure that the scans from each card are synchronised?

Adding another card, however, is an overkill.  What I want to do is to measure Acoustic Emission signal in one channel and a displacement in the other.  I'm interereseted in AE signal up to 600 kHz but the displacement sine wave is 1-3 Hz.

What I need is to be able to measure both at the same time, in order to be able to interpret the AE signal better later on.

There must be a cheaper solution, yet reliable.  E.g. isn't it possible to syncronize the serial port with the card? If not, what about the USB port?

Regards,
Runson

[Tesla1977]

Seems to me that you simply have the wrong card for your purpose.

There are simultaneous-sampling devices from NI, if your card is quite new try to give it back and get a device fitting your requirements.

[Lorne H.]

Runson,

You are restricted by your need to synchronize your two measurements.  But you do have two good options to choose from.

Option 1 (What I would recommend):

Purchase an additional M series DAQ board.  I would purchase the PCI-6220 since you do not need as high of a sampling rate to measure your displacement sine wave.  Then, connect your 6220 to your 6250 using a RTSI cable.  The RTSI cable will allow you to share the sample clock between the two boards enabling you to synchronize your measurements.

Option 2:

Purchase an S-series board.  The PCI-6120 has a sampling rate 1MS per channel for 4 channels and unlike the 6250 you can sample on all of the channels at the same time at the max sampling rate.

I hope this helps,
Lorne Hengst
Application Engineer
National Instruments

[runson]

Ok now we're onto something ...

Can I use an E-Series board for this?  I can get my hands on PCI-6024E

Regards,

Runson

[Lorne H.]

Runson,

You should be able to do that as long as you purchase the RTSI cable.

Lorne Hengst
Application Engineer
National Instruments