Earlier last month I asked a question regrading pushing non-cyclic buffers using ADALM2000 and the fix on the DAC non-cyclic push has helped us a lot in a project we are currently working on. Although I still noticed some weird behaviors of the DAC output voltage by observing the pushed data using the oscilloscope, I didn't raise a question immediately since it was not affecting our project. However, this week as we try to move our ADALM-based communication system from baseband to RF band, this DAC voltage issue has become inevitable and I would like to ask you guys for help!
The issue is, although after the last fix on pushing non-cyclic buffer we are able to capture clean OOK signals using the oscilloscope, there are some weird voltage variations (fluctuations) after the analog output is enabled and before the first buffer (we call it packet in our project) is sent, as you could see from the oscilloscope screenshot attached below. There are 3 undesired pulses with voltage at around 200 mV. And after the three pulses we began to observe the actually pushed buffers with 400 mV voltage. In fact, we have chose not to calibrate the DAC output since if we calibrate there will be pulses of even higher voltage occurring before the first buffer is sent.
This was previously not an issue since we were sending 1V amplitude OOK signals and triggered at 0.8V (the pulse peaks, as you could see, are only 0.4 V). But now as we transition into the RF band we have to limit the analog out voltage to 0.35 V and these pulses become a problem (e.g., causing unwanted triggering at the beginning of the data transmission). Is there any chance you guys could get rid of these pulses occurring before the first pushed buffer?
Thank you so much for your help!
(P.S.) Another quick question: Is it normal to observe about 20mV of noise when connecting ADAML with the USB port of the computer while not running any programs? As we move our project to the RF we were wondering the noise floor of ADALM's analog input. After connecting the ADALM with the laptop, we hooked up the oscilloscope probes with the analog "input" of the ADALM. We observed about 20 mV amplitude noise from the oscilloscope (without running any programs on ADALM except that it is connected with the computer) and were wondering if this is expected (normal). If so I guess we will try amplifying our RF input voltage instead.
Captured oscilloscope output showing 3 peaks of 200 mV before pushing the non-cyclic OOK signals of 400 mV.
Below is the zoomed-in version of the oscilloscope screen:
ADALM is connected with the laptop via the USB and the analog input 1 and ground are connected with the osciiloscpe.
Observed oscilloscope output, the noise voltage was much lower when ADALM is not connected with the PC/powered on.
I figured out where the pulses are coming from. Unfortunately both ADC and DAC calibration rely on the DAC outputs to calibrate. And because of hardware considerations, we couldn't isolate…