glitches in propscope outputs from DAC.

waltsailing

Hi,

I just received the prop scope. Very cool but, maybe it has a problem. I set it up to output a signal from the DAC to CHN 2. It was set to a simple sine wave. It has these unexpected glitches in it. Happens at many different frequencies and sampling speeds and at different levels. I am not using any probes, just using the built in loop around method internal to the propscope on channel 2 where you can select the DAC/signal generator. The glitches show up at differet times and have different values, but appear similiar.

I tried it on a different computer, it did the same thing, so i may have ruled out the power from the USB.

Looks like some sort of timing problem in the unit. Do these all do this?

http://www.youtube.com/watch?v=Z4LEqNSCr0g

This is a short clip showing the problem I am having. The unit is new and the program was downloaded fresh today. Are there any updates to correct this?

I contacted tech support but have not received any answers back from them yet, so i figured i would ask here.

Does the DAC output clean sinewaves at 1 Mhz? What about at say 300 Khz? At 1 Mhz, in addition to the glitches there appears to be some sort of shift in the signal or timing problem. You can see this in the enclosed photo.

Thank you,
Walt,

Hanno

Hi Walt,
Thanks for the feedback!

Your PropScope's DAC output should be free of glitches- please confirm by connecting the DAC output the CH1 analog input and view the CH1 signal.

I added the capability to view the data sent to the DAC at a user request- the intent was to make it easier to measure things like filters. Instead of measuring the actual electronic signal, the internal DAC signal is taken from the INA register, which should reflect the status of the pins that drive the DAC. This introduces two artifacts. First, there's a certain time lag between when data is sent from the Propeller and when the electrical signal is available at the BNC connector. This has been noted before, but since every wire has this problem (light/electricity travels ~30cm/nanosecond) you can work around it. The second issue, which you raised, is caused when the INA register does not accurately reflect the state of the IO pins. As far as I understand, this can happen when the OUTA register is changed at the same time as the INA register is read. Unfortunately changing the timing of OUTA writes and INA reads would introduce other artifacts- either introducing jitter into the function generator or affecting the analog trigger performance- both of which I consider worse than the current situation.

The recommended workaround is to measure the DAC's output with CH1 when you want to accurately measure the actual signal being output by the DAC. Use the internal DAC when you're using CH1 for something else- but keep in mind that there will be some lag and that the internal DAC may show glitches which don't appear in the actual output.
Hanno

waltsailing

Hi Hanno,

Thank you for your suggestion. I have decided to look at this only on channel 1 to understand the limitations, capabilities of the circuits. That appears to have cleared up part of the problem. I then reviewed the preliminary schematic and noted you are using an 8 bit 74AHC541 buffer as the driver into the R2R network for the DAC. This may be related to part of the problem I am seeing. I still get the glitch.

Some of the problem appears to be due to the DAC. Specifically, I turn down the output level and then adjust the offset, I can find places where I think that the DAC is being driven from a state of 127 to a state above 128. This is when the MSB is changing, the R-2R network would give the largest voltage error at this transition, because the output from the 541 goes from 0111,1111 to 1000,0000 and this transition is the mid point of the range of DAC outputs, depending on the offset selected and the voltage scale desired because of the two different output opamps utilized. Errors in the R2R and the variation in drive voltage from the 74AHC541 over different binary input conditions can couple into the outputs of this type of DAC and show the behaviour I think I am seeing. If I then step the offset voltage I can see the signal glitch move in position when this transition from 127 to 128 happens. Makes me think that there might be a minor problem in the tollerances of the resistors on the board, or the power to the 74AHC541 is drooping during this transition. Once it has settled it seems to be fine. So this may be just a settling issue with MSB transistions.

I tried it with a 1 volt signal, zero offset, same issue with the glitch. This is making me think there is either a limitation of the DAC design, or bad resistors on my board...and not the output driver amps which switch between -1.5/1.5 and 0-5 via the bit P11 changes.

I have not gotten out a sperate scope to try this all out yet and debug it, so maybe it is a timing/settling issue when this bit transitions. But it looks like a settling issue.


http://www.youtube.com/watch?v=WLbQ7ahfUWg

The above link is to a youtube video showing the problem.

It is also captured in the enclosed jpg files. Please take a look at these and the youtube video showing the problem.

If it were all a timing problem, then it would appear all over the place and it does not seem to do that and when i controlled the offset voltage I can move the position of the glitch in the waveform, but it seems to stay at the point where I think the MSB is changing.

If you repeat this test, do you see this same type of problem? Or is there something wrong with my new propscope.

I was wondering if you could post the updated DAC card schematics. I found a preliminary one on the forum, and an updated one for the propscope, but no updated one for the DAC card.

Thank You for your assistance,

Regards,
Walt,