USB-205, digital sample rate

[MartinG]

Hello MCCDAQ,

I just ordered and received a USB-205 device.  I intend to use it for validating a complex power sequence for an FPGA design, measuring approx 10 voltages at approx 10Ksamples/sec for approx 100ms - you could say that I want to use the USB-205 as a fairly slow oscilloscope with many, many channels...  Of course the USB-205 "only" has 8 analog inputs, but I intend to use a few of the digital inputs to get around this - some of the supply voltages I want to measure are 3.3V so are compatible with the digital inputs, and I am mainly interested in investigating timing, not specific voltage levels (I have other tests for that).  I intend to use whatever capture/logging software is available, possibly exporting data to e.g. Excel for presentation purposes if that makes sense eventually.

I do understand that the maximum analog sample rate of the USB-205 is 500Ksamples/sec, divided by the number of enabled analog inputs, so in my case 500K/8 inputs = 62.5Ksamples/sec - leaving reasonable headroom for my 10Ksamples/sec requirement (that could probably be relaxed if need be...).  But it seems that the digital sample rate is limited to 50-100 samples/sec - 50 samples/sec in DAQami, 100 samples/sec in TracerDAQ.  I had expected digital sample rate to be at least comparable to analog sample rate?!?

Is there a technical reason for the low digital sample rate - or in other words, might it be circumvented e.g. by accessing the USB-205 "bare-metal" through a script or custom software, instead of relying on the DAQami/TracerDAQ off-the-shelf software?

Cheers, Martin

[JRys]

Hey Martin,

We ran some test here and the best we could achieve with a custom program was about 200 Hz. It just wasn't designed to deliver high speed digital data. We do have a few devices that can read the port faster (with a custom program), but even they won't hit 10,000 Hz. The best device to use to scan a digital port is the USB-DIO32HS. https://www.mccdaq.com/usb-data-acquisition/USB-DIO32HS-Series

Adding a second USB-205 and a custom program is the way to go. With a custom program you could uses the Ext Clock pins on each device and run them synchronously. This way, each channel on both devices is sampled at the sample time. 

Best regards,

John

[JRys]

Hello Martin,

The USB-205 has an oscillator circuit that provides an accurate clock to pace the acquisition, but it only applies to the analog inputs. Reading the digital port is done one update at a time using a software polling scheme, which is much slower operation.

Best regards,

John

[MartinG]

Hi John,

Thanks for your prompt response! :-)

I take it then that the limit at 50 or 100 samples per sec is system dependent and a bit on the safe side from DAQami/TracerDAQ, so I might try upping it with custom software/script running on a dedicated & beefy computer?  Maybe this won't take me all the way to 10Ksamples/sec, less will also work for me - but 10Ksamples/sec really isn't that aggressive, it can even be achieved through an I2C GPIO expander (at least at 400kbits/s ;-).

I will update this post, e.g. with achievable performance numbers, if I ever get to it - but possibly I will just add a second USB-205 to my tool belt for the additional channels.

Again, thanks for your support, much appreciated.

Cheers, Martin