I want to sample 12 V channels simultaneously. Four V channels corresponds to 30 Hz signals, and the rest of them are continuous. I want to sample the AC signals every minute and the CC signals every 10 minutes.

I don't want to get instantaneous samples as this will need both high memory resources and high sampling speeds. However, it is very important that the taken samples give a good average of these signals.

As it is said in the user's manual, each sample (or final reading) derives from four fundamental samples, and each of these last ones is the average of 80 background measurements taken in every mains cycle (e.g. 50 Hz).

Questions:

1. Is the dataTaker taking background samples at 4 kHz (80*50)? So in my case will the final reading for the AC signals (sampled every minute) be the average of 4 fundamental samples.

2. Will each of these last ones be the average of 1200 background samples (15*80)? In this case the taken samples would give a reliable average for the 30 Hz signals?

Thanks in advance and kind regards,
Francisco Jose Munoz Rodriguez
Departamento de Ingeniería Electrónica
de Telecomunicacion y Automatica
Escuela Politécnica Superior. Universidad de Jaén (Spain)
Campus Las Lagunillas 23071

Good morning Francisco,

I'm not exactly sure what the question is here.

When in Normal mode (AKA Continuous mode)the DT800 has the AtoD speed set to 50 kHz.
For a single DC Voltage reading The DT800 samples the analog inputs + to -, + to -, + to Ground and - to Ground. From these four fundamental samples the corrected Voltage is calculated.

To reject the main noise on the system we then repeate this process 80 times over a 20 mSec period and report the average value. So each single Voltage reading is the result of 320 individual AtoD samples.

We also sample up to 5 channels at teh same time by interleaving the readings.

The VAC channel type uses a different method to sample the AC signal.
The Atod is set to read the AC Voltage at a set speed (Default 5 kHz)for a set duration 100 mSec. (four cycles of 50 Hz). The VAC RMS is then calculated from these samples. It should be noted that while sampling an AC voltage the AtoD can not be used to sample any other channels and will measure each AC Voltage channels in listed order.

Cheers,
Roger

Good morning Roger,

Thanks for your help. First of all, sorry if my questions are so innocent but I have recently begun to work with DT800 dataTakers. I'm not sure if the DT800 is taking 80 background samples within each and every cycle of the local mains electricity (i.e. DT800 is always taking background samples) or if it takes them within only one mains period when it is going to get the corresponding fundamental sample.

I suppose that the background samples are taken only when the dataTaker is going to get the final reading but the user's manual is a little bit confusing when it says (page 49 The DT800 takes 80 background measurements within each cycle of the local mains electricity). Can you confirm it?

Another thing and this is my real problem, I want to sample a pulse width modulated, PWM, signal (an asymmetrical square waveform which has a DC component) which can range between 0mV and 200 mV.

This signal has a fixed frequency of 30 Hz. TON and TOFF can vary at least every minute. Please see figure indicated below. Considering these conditions I would like to know how to sample every minute getting information about the RMS or average value of this waveform. The DT800 can provide the RMS voltage of any AC waveform within the frequency range 50 Hz -10 kHz.

The problems are that this signal has both DC and AC components and its frequency is 30 Hz. Could you tell me what can I do?

Thanks in advance,
Francisco

Good morning Francisco,

Always ask questions, it's how we learn.

The AtoD does not sample all the time. The DT800 simply samples the channel for 1 mains period when requested.

The DT800 is not set up to read PWM. You can get an indication of what the PWM is doing by putting a capacitor across the analog input and simply reading the voltage level.

Cheers,
Roger

Good morning Roger,

Thanks again for your help and interest. I had already thought about the idea given in your last email (i.e to use an integrator or an integrated circuit that could give me the RMS value) but I expected the dataTaker to solve this problem without the use of any auxiliary circuit. But as it is said, Nobody and nothing is perfect...

Thanks and kind regards,
Francisco