| Oscilloscope Mode with the DT800 |
Data Electronics DT800 is a very capable and versatile A-D converter, but in most sampling modes it is not particularly fast and that -- coupled with the limits of serial interface speeds -- limits its utility in Oscilloscope Mode. |
By using the 'high speed' setting with mains filtering switched off (see this page), the DT800 can sample at up to about 50-55 readings per second on all channels combined. That's as fast as you can continuously sample in single-channel oscilloscope mode. In Scope + Chart Mode, the maximum sampling rate for the 'scope channel is roughly 55 samples/second divided by the total number of channels. So if you have one 'scope channel and one chart channel, the maximum sampling rate is about 25 samples/second, with further decreases if you add more chart channels up to the maximum of 4.
The DT800 also has a very fast 'Burst' mode, which is automatically used by LabHelper if you select a scope sample rate faster than 60 samples/second. Burst mode requires intermittent sampling, in which the DT800 rapidly takes a 'burst' of samples, stores and processes them internally, and then transmits them to the computer over the serial interface. The latter is the weak link in the process: for example, sending about 1000 burst samples to the computer -- no matter how rapidly they were obtained during the burst -- will take roughly 4-6 seconds. For example, if you were burst sampling 1024 readings at 1000 hertz (1000/second), you could sample about once every 5-7 seconds (1 second to read the burst and 4-6 seconds to send it to the computer). So what you see on the screen is a 'snapshot' of what happened several seconds earlier, not a 'real-time' display of data.
An additional complication in burst mode is that the minimum burst rate is 1000 Hz (1 kHz). This means that if you want a slower rate than 1 kHz, but faster than about 50 Hz, the DT800 has to take multiple burst readings (at 1 kHz) for each sample, which then are down-sample averaged by LabHelper. So if you use 125 Hz and want 1000 samples, the DT800 must make 8000 readings, all of which have to be sent to the computer for processing -- and that will take 8 times as long as it would for 1000 samples at 1 kHz or faster.
After you halt acquisition in burst mode, LabHelper will flush (delete) any data in the DT800's buffer. Doing this removes what amounts to a time lag that would occur if data were retained in the buffer. Flushing opens this window, and as it says, the process can take many seconds if a lot of readings are in the buffer. If the buffer is very full, this window may open several times after you halt acquisition.
Voltage conversion is possible in Oscilloscope mode but only linear conversions are allowed, e.g., result = a + b * volts.All of this makes Scope + Chart mode with the DT800 essentially unuseable at scope sampling rates requiring intermittent sampling, so these speeds are not allowed. DT800's work fine in continuous 'scope mode if your oscilloscope channel needs to work at only 10-50 samples/second. Unless you need very fast sampling and are OK with intermittent readings, the Sable UI2 is a better option for oscilloscope operations.