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max. fast sampling rate of 2204A

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max. fast sampling rate of 2204A

Postby an2flyer » Wed Feb 21, 2018 7:49 am

Hello,

I am controlling a 2204A via SDK and C#

We need to get a single sample at a certain time. So the whole process is invoked by the PC who knows the timing.
We use fast streaming with "ps2000_run_streaming_ns"
Because we need just one single sample, I found out the minimum for parameter "max_samples" is 2 in order to provide an interval for the scope.
This works all fine basically.

However in order to catch the sample as fast as possible I wanted to have the smallest sample rate possible.
Due to the capabilities of the unit 2204A I expected 20ns.
"sample_interval" = 20
"time_units" = Imports.ReportedTimeUnits.NanoSeconds
However this doesn´t work, the call "ps2000_run_streaming_ns" returns zero in this case.

It works first with 300ns which I found out by try and error.

Why does "ps2000_run_streaming_ns" not except sample rates faster than 300ns even though the scope can do this?

Any help is much appreciated
Andy
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Re: max. fast sampling rate of 2204A

Postby Martyn » Wed Feb 21, 2018 9:34 am

Streaming mode is limited to 6.6MS/s or 150nsec sample interval as the data needs to be sent over the USB bus. If you have two channels enabled this will fall to 3.3MS/s or 300nsec sample interval.

I am not sure why you are looking for a single sample, so can you explain a little more about what you are trying to achieve. We may be able to suggest a different approach.
Martyn
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Re: max. fast sampling rate of 2204A

Postby an2flyer » Wed Feb 21, 2018 11:36 am

Hello Martyn,

Thank you very much for your quick and very helpful answer!

There are indeed 2 channels enabled, because the program is directly derived from the PS2000CSConsole
example. So I can expect 150ns with one channel...

Why 1 sample?
I need to test a device out of the automotive industry. This device holds a navigation display which is normally folded in. Using an instruction on the LIN-Bus, a servo motor unfolds the display, so the user can look to it.
During this unfolding process I get position reports from the servo motor over the LIN-Bus every 10ms.
For quality assurance I have to measure the actual current for the servor motor at every position report.
(If there are mechanical problems of any sort, the current curve will show significant variations in this case!)
This measurement of the current is done via a shunt resistor and one channel of the picoscope. Because the range of the scope can be very small (50mV) we even have the benefit of not needing an amplifyer at the shunt!
So with simple words:
When the PC gets a position report, he then requests the actual current via a fast sampling instruction to the picoscope. The value for the current is then stored together with the position.
After the testing the PC will be able to draw a nice diagramm, showing the current in respect to the position of the motor.
So you can see that the timing schedule is controlled via THE PC and not by the picoscope! Therefore we do not need any consecutive samples in this application.

Best regards to you
Andy
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Re: max. fast sampling rate of 2204A

Postby Martyn » Wed Feb 21, 2018 1:51 pm

The issue you will ghave with trying to request samples to match a 10ms time interval is that the commands will need to go from the application, to the driver, and then onto the scope. As this involves the USB bus which is under the control of the operating system the timings can be anywhere between 1ms and 60ms.

To achieve what you require I would be looking at continuously streaming data, throwing away the majority of samples in the callback, and just recording those that hit every 10ms. The exact samples you need to store can be determined from the sample rate.
Martyn
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