High speed logic capture with deep memory

[nlbutts]

I bought a Analog Discovery 3 as a "cheap" alternative to a Saleae. But one problem I am running into is high speed logic capture with deep memory (or fast offload). With the Saleae, I can set a capture rate of 50 Msps of logic and stream it to the PC for minutes. This allows high speed capture of signals, where I can zoom in and inspect the waveforms or search for patterns. I have not found a way of doing that with the Analog Discovery 3. I think the answer is: it can't do it. Does anyone have any suggestions? I've tried all of the various modes and read the manual:

https://digilent.com/reference/test-and-measurement/guides/waveforms-logic-analyzer

[JColvin]

Hi @nlbutts,

I don't have enough data on hand about your setup to immediately be able to give a yes or no answer, so instead I'm going to list out the limiting factors in general with regards to recording digital data with the Analog Discovery 3 and what you can do to mitigate it.

• On-device buffer size. The Analog Discovery 3 does not have any on-board DDR memory to help out with a large number of samples. You can select configuration 4 or 5 in the WaveForms Device Manager as both of those configurations allocate additional sample buffer to the Logic Analyzer.
• Data connection with the host computer. The Analog Discovery 2 uses a USB 2.0 connection. As USB is a packet based protocol, the protocol overhead of completing packet transfers will quickly put a damper in data streaming rates, particularly if you filling up the on-board buffer so fast that the USB protocol doesn't complete its overhead fast enough to empty the buffer again for the device.
• How often the incoming digital data is changing. Like the Saleae (link to Saleae article I'm basing my information off of), the WaveForms software has the option to compress the digital data via run-length encoding (rather than recording a bunch of zeroes in a row, compress the zeros into an indicator of how many zeroes were present before the next detected change in the digital data). The more frequently the in incoming protocol/pattern/bus has a change in the data, the faster the on device buffer will fill up (leading to the above issue).

So I guess my questions to you would be:

- What rate is the incoming data changing at (mostly likely going to be the clock signal if you are recording that)
- How many digital lines are you recording
- Which configuration of the Analog Discovery are you using
- Are you recording to RAM (record button) or to file ("Rec." button next to Export and underneath File / Control / View / Window)

These two other Forum threads might be of interest to you as well:

Thanks,
JColvin

[nlbutts]

So I want to measure a very fast signal, it has a pulse duration of 40 ns (25 MHz). But I want to log data for several seconds. I can either log data at 50 MHz sample rate for a very short time, a few microseconds, or I can log several seconds of data at 100 Hz. But I can't log several seconds of data at 50 MHz. The Analog Discovery 3 seems to enumerate as a USB 3.0 device at 5000 Mbps. It should be able to stream data to the PC and keep up with a fairly high sampling rate. 

I just can't seem to figure out how to configure the GUI to do that. So that is my question, is the hardware and software even capable of streaming captured data to the PC?

[attila]

Hi @nlbutts

The AD3 USB is high-speed, 480MHz.
In the Logic Analyzer the Record mode can be used with data compression which in the WF app is limited to 256Mi samples, 5.3s @ 50MHz. To prevent device buffer overflow the average signal toggle rate should be below ~5MHz.
The gaps between individual capture in Repeated mode can be a few milliseconds, depending on the processing performed, added interpreters, opened views...

If you are only interested in the event (trigger) rate and count you can use the Counter view which measures the trigger timing/rate in the device:

 

The ADP3X50 has much deeper DDR-RAM buffer and supports device buffering (memory segmentation) in Scope and Logic Analyzer which can perform sequence of captures at low latency (<1us) up to 32768, depending on the enabled channels and specified samples to collect for each capture, 64M-128M / Samples / channels